Search: "n distinct prime factors"
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A180278
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Smallest nonnegative integer k such that k^2 + 1 has exactly n distinct prime factors.
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+20
15
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0, 1, 3, 13, 47, 447, 2163, 24263, 241727, 2923783, 16485763, 169053487, 4535472963, 36316463227, 879728844873, 4476534430363, 119919330795347, 1374445897718223, 106298577886531087
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OFFSET
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0,3
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COMMENTS
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LINKS
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FORMULA
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EXAMPLE
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a(2) = 3 because the 2 distinct prime factors of 3^2 + 1 are {2, 5};
a(10) = 16485763 because the 10 distinct prime factors of 16485763^2 + 1 are {2, 5, 13, 17, 29, 37, 41, 73, 149, 257}.
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MATHEMATICA
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a[n_] := a[n] = Module[{k = 1}, If[n == 0, Return[0]]; Monitor[While[PrimeNu[k^2 + 1] != n, k++]; k, {n, k}]]; Table[a[n], {n, 0, 8}] (* Robert P. P. McKone, Sep 13 2023 *)
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PROG
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(Python)
from itertools import count
from sympy import factorint
return next(k for k in count() if len(factorint(k**2+1)) == n) # Pontus von Brömssen, Sep 12 2023
(PARI) a(n)=for(k=0, oo, if(omega(k^2+1) == n, return(k))) \\ Andrew Howroyd, Sep 12 2023
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CROSSREFS
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KEYWORD
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nonn,hard,more
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AUTHOR
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EXTENSIONS
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a(9), a(10) and example corrected; a(11) added, Donovan Johnson, Aug 27 2012
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STATUS
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approved
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A060319
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Smallest Fibonacci number with n distinct prime factors.
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+20
14
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1, 2, 21, 610, 6765, 832040, 102334155, 190392490709135, 1548008755920, 23416728348467685, 2880067194370816120, 81055900096023504197206408605, 2706074082469569338358691163510069157, 5358359254990966640871840, 57602132235424755886206198685365216, 18547707689471986212190138521399707760
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OFFSET
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0,2
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LINKS
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EXAMPLE
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a(5) = F(30) = 832040 = 2^3 * 5 * 11 * 41 * 61.
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MATHEMATICA
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f[n_]:=Length@FactorInteger[Fibonacci[n]]; lst={}; Do[Do[If[f[n]==q, Print[Fibonacci[n]]; AppendTo[lst, Fibonacci[n]]; Break[]], {n, 280}], {q, 18}]; lst (* Vladimir Joseph Stephan Orlovsky, Nov 23 2009 *)
First /@ SortBy[#, Last] &@ Map[#[[1]] &, Values@ GroupBy[#, Last]] &@ Table[{#, PrimeNu@ #} &@ Fibonacci@ n, {n, 2, 200}] (* Michael De Vlieger, Feb 18 2017, Version 10 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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A060320
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Index of smallest Fibonacci number with exactly n distinct prime factors.
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+20
10
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1, 3, 8, 15, 20, 30, 40, 70, 60, 80, 90, 140, 176, 120, 168, 180, 324, 252, 240, 378, 450, 432, 552, 360, 420, 690, 504, 880, 630, 600, 756, 720, 900, 792, 840, 1296, 1050, 1350, 1140, 1080, 1200
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OFFSET
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0,2
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COMMENTS
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a(42) = 1260, a(44) = 1320; for all other n > 40, a(n) > 1350.
These results were computed using data at the Blair Kelly site under Links at A022307. Note that the presence of incompletely factored Fibonacci numbers with indices as low as 1301 does not prevent the drawing of conclusions such as "a(44) = 1320" with certainly. Using F(1301) as an example, the compact table of Fibonacci results at the Kelly site indicates that F(1301) = p*q*r*c where p=6400921, q=14225131397, r=100794731109596201, and c is a 238-digit unfactored composite number. The complete factorization of every Fibonacci number up to F(1000) is explicitly given elsewhere on the site, and those results allow quick verification that a(n) <= 900 for all n in [0..34], so 1301 cannot be a term unless F(1301) has at least 35 distinct prime factors, which would require c to have at least 32 distinct prime factors, at least one of which would have to be less than ceiling(c^(1/32)) = 26570323, but trial division of c by every prime less than 26570323 shows that c has no prime factors that small. Thus, while A022307(1301) is unknown, it is certain that 1301 is not a term in this sequence. Similarly, making use of known factors, it can be proved that F(n) cannot have 44 or more distinct prime factors for any n < 1320, so since F(1320) has exactly 44 distinct prime factors, it is established that a(44) = 1320. (End)
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LINKS
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FORMULA
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EXAMPLE
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n=9: F(80) = 23416728348467685 = 3 * 5 * 7 * 11 * 41 * 47 * 1601 * 2161 * 3041.
n=25: F(690) = 2^3 * 5 * 11 * 31 * 61 * 137 * 139 * 461 * 691 * 829 * 1151 * 1381 * 4831 * 5981 * 18077 * 28657 * 186301 * 324301 * 686551 * 1485571 * 4641631 * 117169733521 * 2441738887963981 * 3490125311294161 * 25013864044961447973152814604981 is the smallest Fibonacci number with exactly 25 distinct prime factors.
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MATHEMATICA
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First /@ SortBy[#, Last] &@ Map[First@ # &, Values@ GroupBy[#, Last]] &@ Table[{n - Boole[n == 2], #, PrimeNu@ #} &@ Fibonacci@ n, {n, 2, 300}] (* Michael De Vlieger, Feb 18 2017, Version 10 *)
Module[{ff=Table[{n, PrimeNu[Fibonacci[n]]}, {n, 1400}]}, Table[ SelectFirst[ ff, #[[2]]==k&], {k, 0, 40}]][[All, 1]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Dec 28 2018 *)
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PROG
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(PARI) my(o=[], s); print1(1); for(n=1, 20, s=0; until( o[s]==n, #o<s++ && o=concat(o, omega(fibonacci(s))) ); print1(", "s))
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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A048692
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Triangle read by rows in which row n contains first n numbers with exactly n distinct prime factors.
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+20
9
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2, 6, 10, 30, 42, 60, 210, 330, 390, 420, 2310, 2730, 3570, 3990, 4290, 30030, 39270, 43890, 46410, 51870, 53130, 510510, 570570, 690690, 746130, 870870, 881790, 903210, 9699690, 11741730, 13123110, 14804790, 15825810, 16546530, 17160990, 17687670, 223092870, 281291010, 300690390, 340510170, 358888530, 363993630, 380570190, 397687290, 406816410
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OFFSET
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1,1
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LINKS
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EXAMPLE
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2;
6, 10;
30, 42, 60;
210, 330, 390, 420;
...
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MATHEMATICA
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f[n_] := Flatten[Table[ # [[1]]] & /@ FactorInteger[n]]; (* for n=7 *) Take[ Select[ Range[10^7], Length[f[ # ]] == 7 & ], 7]
Module[{nn=8, dpf=Table[{n, PrimeNu[n]}, {n, 2 10^7}]}, Flatten[Table[Select[dpf, #[[2]]==n&, n], {n, nn}], 1][[All, 1]]] (* The program generates the first 36 terms of the sequence. *) (* Harvey P. Dale, Sep 09 2022 *)
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CROSSREFS
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Extending the rows to give a square array, we get A125666.
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KEYWORD
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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A067024
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Smallest prime p such that p+2 has exactly n distinct prime factors.
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+20
8
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2, 13, 103, 1153, 15013, 255253, 4849843, 111546433, 4360010653, 100280245063, 5245694198743, 152125131763603, 7149881192889433, 421842990380476663, 16294579238595022363, 1106494163767990292293, 74135108972455349583763, 4632891063696575353839163, 278970415063349480483707693, 24012274383139350058948392193
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OFFSET
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1,1
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LINKS
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FORMULA
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EXAMPLE
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For n = 1,...,7 the factors of 2+a(n) are as follows: 2*2, 3*5, 3*5*7, 3*5*7*11, 3*5*7*11*13, 3*5*7*11*13*17, 3*5*7*11*13*17*19; i.e., a(n) = A002110(n+1)/2 which is prime for n = 2,...,7.
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PROG
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(Python) # see linked program
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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A242621
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Start of the least triple of consecutive squarefree numbers each of which has exactly n distinct prime factors.
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+20
8
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OFFSET
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1,1
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COMMENTS
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As the example of a(4)=27962 shows, "consecutive squarefree numbers" means consecutive elements of A005117, not necessarily consecutive integers that (additionally) are squarefree; this would be a more restrictive condition.
a(8) <= 102099792179229 because A093550 - 1 is an upper bound of the present sequence.
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LINKS
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EXAMPLE
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The two squarefree numbers following a(4)=27962, namely, 27965 and 27966, also have 4 prime divisors just as a(4).
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CROSSREFS
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See A242605-A242608 for triples of consecutive squarefree numbers with m=2,..,5 prime factors.
See A246470 for the quadruplet and A246548 for the 5-tuple versions of this sequence.
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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A156329
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Smallest tetrahedral number with n distinct prime factors.
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+20
7
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4, 10, 84, 1140, 7140, 125580, 2929290, 161226780, 2875585020, 32451432090, 9117204216120, 173092525291140, 12728365882372140, 6235727798083743960, 843456728066008506450, 68313970807402942762140, 7219596660397839675355860, 211677272048242059920400540
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OFFSET
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1,1
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LINKS
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EXAMPLE
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a(9) = 2875585020 = 2^2*3*5*7*11*17*19*41*47. 2875585020 is the smallest tetrahedral number with 9 distinct prime factors.
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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A188342
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Smallest odd primitive abundant number (A006038) having n distinct prime factors.
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+20
7
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945, 3465, 15015, 692835, 22309287, 1542773001, 33426748355, 1635754104985, 114761064312895, 9316511857401385, 879315530560980695, 88452776289145528645, 2792580508557308832935, 428525983200229616718445, 42163230434005200984080045, 1357656019974967471687377449
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OFFSET
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3,1
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COMMENTS
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Dickson proves that there are only a finite number of odd primitive abundant numbers having n distinct prime factors. For n=3, there are 8 such numbers: 945, 1575, 2205, 7425, 78975, 131625, 342225, 570375. See A188439.
a(15) <= 2792580508557308832935, a(16) <= 428525983200229616718445, a(17) <= 42163230434005200984080045. If these a(n) are squarefree and don't have a greatest prime factor more than 3 primes away from that of the preceding term, then these bounds are the actual values of a(n). The PARI code needs only fractions of a second to compute further bounds, which under the given hypotheses are the actual values of a(n). - M. F. Hasler, Jul 17 2016
It appears that the terms are squarefree for n >= 5, so they yield also the smallest term of A249263 with n factors; see A287581 for the largest such, and A287590 for the number of such terms with n factors. (For nonsquarefree odd abundant numbers, this seems to be known only for n = 3 and n = 4 prime factors (8 respectively 576 terms), cf. A188439.) - M. F. Hasler, May 29 2017
"If these a(n) are squarefree and don't have a greatest prime factor more than 3 primes away from that of the preceding term, then these bounds are the actual values of a(n)."
This conjecture is correct up to a(50). (End)
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LINKS
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EXAMPLE
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945 = 3^3 * 5 * 7
3465 = 3^2 * 5 * 7 * 11
15015 = 3 * 5 * 7 * 11 * 13
692835 = 3 * 5 * 11 * 13 * 17 * 19 (n=6: gpf increases by 2 primes)
22309287 = 3 * 7 * 11 * 13 * 17 * 19 * 23
1542773001 = 3 * 7 * 11 * 17 * 19 * 23 * 29 * 31
33426748355 = 5 * 7 * 11 * 13 * 17 * 19 * 23 * 29 * 31
1635754104985 = 5 * 7 * 11 * 13 * 17 * 19 * 23 * 29 * 37 * 41 (here too)
114761064312895 = 5 * 7 * 11 * 13 * 17 * 23 * 29 * 31 * 37 * 41 * 43
9316511857401385 = 5 * 7 * 13 * 17 * 19 * 23 * 29 * 31 * 37 * 41 * 43 * 47
879315530560980695 = 5 * 7 * 13 * 17 * 19 * 23 * 29 * 31 * 37 * 41 * 53 * 59 * 61 (n=13: gpf increases for the first time by 3 primes) (End)
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MATHEMATICA
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PrimAbunQ[n_] := Module[{x, y},
y = Most[Divisors[n]]; x = DivisorSigma[1, y];
DivisorSigma[1, n] > 2 n && AllTrue[x/y, # <= 2 &]];
Table[k = 1;
While[! PrimAbunQ[k] || Length[FactorInteger[k][[All, 1]]] != n,
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PROG
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(PARI) A188342=[0, 0, 945, 3465]; a(n, D(n)=n\6+1)={while(n>#A188342, my(S=#A188342, T=factor(A188342[S])[, 1], M=[primepi(T[1]), primepi(T[#T])+D(S++)], best=prime(M[2])^S); forvec(v=vector(S, i, M), best>(T=prod(i=1, #v, prime(v[i]))) && (S=prod(i=1, #v, prime(v[i])+1)-T*2)>0 && S*prime(v[#v])<T*2 && best=T, 2); A188342=concat(A188342, best)); A188342[n]} \\ Assuming a(n) squarefree for n>4, search is exhaustive within the limit primepi(gpf(a(n))) <= primepi(gpf(a(n-1)))+D(n), with D(n) given as optional 2nd arg. - M. F. Hasler, Jul 17 2016
(PARI)
generate(A, B, n) = A=max(A, vecprod(primes(n+1))\2); (f(m, p, j) = my(list=List()); if(sigma(m) > 2*m, return(list)); forprime(q=p, sqrtnint(B\m, j), my(v=m*q); while(v <= B, if(j==1, if(v>=A && sigma(v) > 2*v, my(F=factor(v)[, 1], ok=1); for(i=1, #F, if(sigma(v\F[i], -1) > 2, ok=0; break)); if(ok, listput(list, v))), if(v*(q+1) <= B, list=concat(list, f(v, q+1, j-1)))); v *= q)); list); vecsort(Vec(f(1, 3, n)));
a(n) = my(x=vecprod(primes(n+1))\2, y=2*x); while(1, my(v=generate(x, y, n)); if(#v >= 1, return(v[1])); x=y+1; y=2*x); \\ Daniel Suteu, Feb 10 2024
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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a(14)-a(17) confirmed and a(18) from Daniel Suteu, Feb 10 2024
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STATUS
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approved
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A188439
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Irregular triangle of odd primitive abundant numbers (A006038) in which row n has numbers with n distinct prime factors.
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+20
7
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945, 1575, 2205, 7425, 78975, 131625, 342225, 570375, 3465, 4095, 5355, 5775, 5985, 6435, 6825, 7245, 8085, 8415, 8925, 9135, 9555, 9765, 11655, 12705, 12915, 13545, 14805, 16695, 18585, 19215, 21105, 22365, 22995, 24885, 26145, 28035, 28215, 29835
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OFFSET
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3,1
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COMMENTS
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The initial row has 8 terms. Row n begins with A188342(n). Dickson proves that each row has a finite number of terms. He lists the first two rows in factored form in his paper. However, as Ferrier and Herzog report, Dickson's tables have many errors. There are 576 odd primitive abundant numbers (OPAN) having 4 distinct prime factors, the last of which is 3^10 5^5 17^4 251^2 = 970969744245403125. The next row, for 5 distinct prime factors, has over 100000 terms.
If the prime factors were counted with multiplicity, then the table would start with row 5, having 121 terms: (945, 1575, 2205, 3465, 4095, ..., 430815, 437745, 442365). Row 6 would start (7425, 28215, 29835, 33345, 34155, ...), and row 7, (81081, 121095, 164835, 182655, 189189, ...). - M. F. Hasler, Jul 27 2016 [See A287646.]
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LINKS
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EXAMPLE
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Row 3: 945, 1575, 2205, 7425, 78975, 131625, 342225, 570375;
Row 4: 3465, 4095, 5355, ...(571 more)..., 249450402403828125, 970969744245403125;
Row 5: 15015, 19635, 21945, 23205, 25935, 26565, 31395, 33495, 33915, 35805, ...
Row 6: 692835, 838695, 937365, 1057485, 1130415, 1181895, 1225785, 1263405, ...
Row 7: 22309287, 28129101, 30069039, 34051017, 35888853, 36399363, ...
The first column is A188342 = (945, 3465, 15015, 692835, 22309287, ...) (End)
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CROSSREFS
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Cf. A006038 (all OPAN), A188342 (first column of this table), A287646 (variant where row n contains all OPAN with n prime factors counted with multiplicity).
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KEYWORD
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nonn,tabf
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AUTHOR
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STATUS
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approved
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A358862
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a(n) is the smallest n-gonal number with exactly n distinct prime factors.
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+20
7
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66, 44100, 11310, 103740, 3333330, 185040240, 15529888374, 626141842326, 21647593547580, 351877410344460, 82634328555218440, 2383985537862979050, 239213805711830629680
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OFFSET
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3,1
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COMMENTS
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The corresponding indices of n-gonal numbers are 11, 210, 87, 228, 1155, 7854, 66612, 395646, 2193303, ...
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LINKS
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EXAMPLE
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a(3) = 66, because 66 is a triangular number with 3 distinct prime factors {2, 3, 11} and this is the smallest such number.
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MATHEMATICA
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Table[SelectFirst[PolygonalNumber[n, Range[400000]], PrimeNu[#]==n&], {n, 3, 10}] (* The program generates the first 8 terms of the sequence. *) (* Harvey P. Dale, Sep 09 2023 *)
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PROG
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(PARI) a(n) = if(n<3, return()); for(k=1, oo, my(t=(k*(n*k - n - 2*k + 4))\2); if(omega(t) == n, return(t))); \\ Daniel Suteu, Dec 04 2022
(PARI)
omega_polygonals(A, B, n, k) = A=max(A, vecprod(primes(n))); (f(m, p, j) = my(list=List()); forprime(q=p, sqrtnint(B\m, j), my(v=m*q, r=nextprime(q+1)); while(v <= B, if(j==1, if(v>=A && ispolygonal(v, k), listput(list, v)), if(v*r <= B, list=concat(list, f(v, r, j-1)))); v *= q)); list); vecsort(Vec(f(1, 2, n)));
a(n, k=n) = if(n < 3, return()); my(x=vecprod(primes(n)), y=2*x); while(1, my(v=omega_polygonals(x, y, n, k)); if(#v >= 1, return(v[1])); x=y+1; y=2*x); \\ Daniel Suteu, Dec 04 2022
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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