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A167147 a(n) = Number of subsets with n members of the set {1..n^2} such that the sum of the members is prime. 4

%I #23 Oct 24 2016 17:11:32

%S 4,26,444,11998,382716,15844060,766387489,42716991304,2704983373484,

%T 193042233338665,15032959574223321,1289808484211222447,

%U 120472472277271660102,12036408131864572935262,1297010265532587186011353,151499235341042432049982767,18434494194245279115211501310,2430305919107872967957571237320,334200348422242729412526022526012

%N a(n) = Number of subsets with n members of the set {1..n^2} such that the sum of the members is prime.

%C The sequence was conceived by _Zak Seidov_.

%C Further terms were calculated by _Alois P. Heinz_.

%H Alois P. Heinz, <a href="/A167147/b167147.txt">Table of n, a(n) for n = 2..29</a>

%e When n=2, a(2) = 4 because there are 4 subsets of the set {1,2,3,4} with prime sums: {1,2}=>3, {1,4}=>5, {2,3}=>5, {3,4}=>7.

%e When n=3, a(3) = 26 because there are 26 subsets of the set {1,2,3,4,5,6,7,8,9} with prime sums: {1,2,4}=>7, {1,2,8}=>11, {1,3,7}=>11, {1,3,9}=>13, {1,4,6}=>11, {1,4,8}=>13, {1,5,7}=>13, {1,7,9}=>17, {2,3,6}=>11, {2,3,8}=>13, {2,4,5}=>11, {2,4,7}=>13, {2,5,6}=>13, {2,6,9}=>17, {2,7,8}=>17, {2,8,9}=>19, {3,4,6}=>13, {3,5,9}=>17, {3,6,8}=>17, {3,7,9}=>19, {4,5,8}=>17, {4,6,7}=>17, {4,6,9}=>19, {4,7,8}=>19, {5,6,8}=>19, {6,8,9}=>23.

%p g:= proc(n, i, t) option remember;

%p if n<0 or t<0 then 0

%p elif n=0 then `if`(t=0, 1, 0)

%p elif i<1 or i<t or (i+(1-t)/2)*t<n then 0

%p else g(n, i-1, t) + g(n-i, i-1, t-1)

%p fi

%p end;

%p a:= proc(n) option remember;

%p add(`if`(isprime(k), g(k, min(k, n^2), n), 0), k=2..n^2*(n^2+1)/2)

%p end:

%p seq(a(n), n=2..13);

%p # Coded by _Alois P. Heinz_

%t g[n_, i_, t_] := g[n, i, t] = Which[n<0 || t<0, 0, n == 0, If[t == 0, 1, 0], i<1 || i<t || (i+(1-t)/2)*t < n, 0, True, g[n, i-1, t] + g[n-i, i-1, t-1]]; a[n_] := a[n] = Sum[If[PrimeQ[k], g[k, Min[k, n^2], n], 0], {k, 2, n^2*(n^2 + 1)/2}]; Table[Print[a[n]]; a[n], {n, 2, 13}] (* _Jean-François Alcover_, Oct 24 2016, after _Alois P. Heinz_ *)

%K nonn

%O 2,1

%A _Christopher Hunt Gribble_, Oct 28 2009

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Last modified March 28 11:46 EDT 2024. Contains 371241 sequences. (Running on oeis4.)