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A228058 Odd numbers of the form p^(1+4k) * r^2, where p is prime of the form 1+4m, r > 1, and gcd(p,r) = 1. (Euler's criteria for odd perfect numbers). 36
45, 117, 153, 245, 261, 325, 333, 369, 405, 425, 477, 549, 605, 637, 657, 725, 801, 833, 845, 873, 909, 925, 981, 1017, 1025, 1053, 1233, 1325, 1341, 1377, 1413, 1421, 1445, 1525, 1557, 1573, 1629, 1737, 1773, 1805, 1813, 1825, 2009, 2057, 2061, 2097, 2169 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

It has been proved that if an odd perfect number exists, it belongs to this sequence. The first term of the form p^5 * n^2 is 28125 = 5^5 * 3^2, occurring in position 520.

Sequence A228059 lists the subsequence of these numbers that are closer to being perfect than smaller numbers. - T. D. Noe, Aug 15 2013

Sequence A326137 lists terms with at least five distinct prime factors. See further comments there. - Antti Karttunen, Jun 13 2019

LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000

Charles Greathouse and Eric W. Weisstein, MathWorld: Odd perfect number

Oliver Knill, The oldest open problem in mathematics, Handout for NEU Math Circle, December 2, 2007

P. P. Nielsen, Odd Perfect Numbers Have At Least Nine Distinct Prime Factors, arXiv:math/0602485 [math.NT], 2006.

Wikipedia, Perfect number: Odd perfect numbers

Index entries for sequences where any odd perfect numbers must occur

FORMULA

From Antti Karttunen, Apr 22 2019 & Jun 03 2019: (Start)

A325313(a(n)) = -A325319(n).

A325314(a(n)) = -A325320(n).

A001065(a(n)) = A325377(n).

A033879(a(n)) = A325379(n).

A034460(a(n)) = A325823(n).

A325814(a(n)) = A325824(n).

A324213(a(n)) = A325819(n).

(End)

MATHEMATICA

nn = 100; n = 1; t = {}; While[Length[t] < nn, n = n + 2; {p, e} = Transpose[FactorInteger[n]]; od = Select[e, OddQ]; If[Length[e] > 1 && Length[od] == 1 && Mod[od[[1]], 4] == 1 && Mod[p[[Position[e, od[[1]]][[1, 1]]]], 4] == 1, AppendTo[t, n]]]; t (* T. D. Noe, Aug 15 2013 *)

PROG

(Haskell)

import Data.List (partition)

a228058 n = a228058_list !! (n-1)

a228058_list = filter f [1, 3 ..] where

   f x = length us == 1 && not (null vs) &&

         fst (head us) `mod` 4 == 1 && snd (head us) `mod` 4 == 1

         where (us, vs) = partition (odd . snd) $

                         zip (a027748_row x) (a124010_row x)

-- Reinhard Zumkeller, Aug 14 2013

(PARI)

up_to = 1000;

isA228058(n) = if(!(n%2)||(omega(n)<2), 0, my(f=factor(n), y=0); for(i=1, #f~, if(1==(f[i, 2]%4), if((1==y)||(1!=(f[i, 1]%4)), return(0), y=1), if(f[i, 2]%2, return(0)))); (y));

A228058list(up_to) = { my(v=vector(up_to), k=0, n=0); while(k<up_to, n++; if(isA228058(n), k++; v[k] = n)); (v); };

v228058 = A228058list(up_to);

A228058(n) = v228058[n]; \\ Antti Karttunen, Apr 22 2019

CROSSREFS

Subsequence of A191218, and also of A228056 and A228057 (simpler versions of this sequence).

For various subsequences with additional conditions, see A228059, A325376, A325380, A325822, A326137 and also A324898 (subsequence if it does not contain any prime powers).

Cf. A027748, A124010, A005408, A324647, A325319, A325320, A325375, A325377, A325378, A325379, A325819, A325823, A325824.

Sequence in context: A044232 A044613 A039528 * A074770 A140369 A044296

Adjacent sequences:  A228055 A228056 A228057 * A228059 A228060 A228061

KEYWORD

nonn

AUTHOR

T. D. Noe, Aug 13 2013

EXTENSIONS

Note in parentheses added to the definition by Antti Karttunen, Jun 03 2019

STATUS

approved

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Last modified January 28 22:40 EST 2020. Contains 331328 sequences. (Running on oeis4.)