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A288814 a(n) is the smallest composite number whose prime divisors (with multiplicity) sum to n. 17
4, 6, 8, 10, 15, 14, 21, 28, 35, 22, 33, 26, 39, 52, 65, 34, 51, 38, 57, 76, 95, 46, 69, 92, 115, 184, 161, 58, 87, 62, 93, 124, 155, 248, 217, 74, 111, 148, 185, 82, 123, 86, 129, 172, 215, 94, 141, 188, 235, 376, 329, 106, 159, 212, 265, 424, 371, 118, 177, 122, 183, 244, 305, 488, 427, 134, 201, 268, 335, 142 (list; graph; refs; listen; history; text; internal format)
OFFSET

4,1

COMMENTS

Agrees with A056240(n) if n is composite (but not if n is prime).

For n prime, let P_n = greatest prime < n such that A056240(n-P_n) = A288313(m) for some m; then a(n) = Min{q*a(n-q): q prime, n-1 > q >= P_n}.

In most cases q is the greatest prime < p, but there are exceptions; e.g., p=211 is the smallest prime for which q (=197) is the second prime removed from 211, not the first. 541 is the next prime with this property (q=521). The same applies to p=16183, for which q=16139, the second prime removed from p. These examples all arise with q being the lesser of a prime pair.

For p prime, a(p) = q*a(p-q) for some prime q < p as described above. Then a(p-q) = 2,4,8 or 3*r for some prime r.

The subsequence of terms (4, 6, 8, 10, 14, 21, 22, 26, 34, ...), where for all m > n, a(m) > a(n) is the same as sequence A088686, and the sequence of its indices (4, 5, 6, 7, 9, 10, 13, 19, ...) is the same as A088685. - David James Sycamore, Jun 30 2017

Records are in A088685. - Robert G. Wilson v, Feb 26 2018

Number of terms less than 10^k, k=1,2,3,...: 3, 32, 246, 2046, 17053, 147488, ..., . - Robert G. Wilson v, Feb 26 2018

LINKS

David A. Corneth, Table of n, a(n) for n = 4..10003 (terms 4..1000 from Michel Marcus)

David A. Corneth, PARI program

EXAMPLE

a(5) = 6 = 2*3 is the smallest composite number whose prime divisors add to 5.

a(7) = 10 = 2*5 is the smallest composite number whose prime divisors add to 7.

12 = 2 * 2 * 3 is not in the sequence, since the sum of its prime divisors is 7, a value already obtained by the lesser 10. - David A. Corneth, Jun 22 2017

MAPLE

N:= 100: # to get a(4)..a(N)

V:= Array(4..N): count:= 0:

for k from 4 while count < N-3 do

  if isprime(k) then next fi;

  s:= add(t[1]*t[2], t = ifactors(k)[2]);

if s <= N and V[s]=0 then

    V[s]:= k; count:= count+1;

fi

od:

convert(V, list); # Robert Israel, Feb 26 2018

MATHEMATICA

Function[s, Table[FirstPosition[s, _?(# == n &)][[1]], {n, 4, 73}]]@ Table[Boole[CompositeQ@ n] Total@ Flatten@ Map[ConstantArray[#1, #2] & @@ # &, FactorInteger[n]], {n, 10^3}] (* Michael De Vlieger, Jun 19 2017 *)

f[n_] := If[ PrimeQ@ n, 0, spf = Plus @@ Flatten[ Table[#1, {#2}] & @@@ FactorInteger@ n]]; t[_] := 0; k = 1; While[k < 500, If[ t[f[k]] == 0, t[f[k]] = k]; k++]; t@# & /@ Range[4, 73] (* Robert G. Wilson v, Feb 26 2018 *)

PROG

(PARI) isok(k, n) = my(f=factor(k)); sum(j=1, #f~, f[j, 1]*f[j, 2]) == n;

a(n) = forcomposite(k=1, , if (isok(k, n), return(k))); \\ Michel Marcus, Jun 21 2017

(PARI) lista(n) = {my(res = vector(n), s, todo); if(n < 4, return([]), todo = n-3); forcomposite(k=4, , f=factor(k); s = sum(j=1, #f~, f[j, 1]*f[j, 2]); if(s<=n, if(res[s]==0, res[s]=k; todo--; if(todo==0, return(vector(n-3, i, res[i+3]))))))} \\ David A. Corneth, Jun 21 2017

(PARI) See PARI-link \\ David A. Corneth, Mar 23 2018

CROSSREFS

Cf. A046343, A056240, A088685, A288313.

Sequence in context: A088686 A161344 A127792 * A062711 A280739 A117347

Adjacent sequences:  A288811 A288812 A288813 * A288815 A288816 A288817

KEYWORD

nonn

AUTHOR

David James Sycamore, Jun 16 2017

STATUS

approved

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Last modified October 18 17:13 EDT 2019. Contains 328186 sequences. (Running on oeis4.)