%I #8 Sep 03 2021 20:41:30

%S 1,7,9,15,17,21,31,27,35,33,39,41,45,47,55,51,53,57,0,63,67,65,71,0,

%T 79,81,0,85,77,83,99,0,0,89,97,95,103,111,101,0,0,0,115,107,0,129,121,

%U 113,0,141,119,0,0,125,133,147,0,131,159,145,153,151,137,0,0,143,0,0,149,155,0,0,0,163,189

%N a(n) is the least odd number that has exactly n decompositions as the sum of three primes, or 0 if there is no such odd number.

%C Entries of 0 are conjectural. If nonzero they are greater than 10^5.

%C Assuming Goldbach's conjecture, if k is odd then A068307(k) >= pi(k-4)-pi((k-1)/2). Using Pierre Dusart's bounds on pi(x), this implies that, for example, A068307(k) >= 4292 for odd k >= 10^5. Thus (on the assumption of Goldbach's conjecture) the given entries of 0 are correct.

%e a(3) = 15 because 15 has exactly 3 decompositions as the sum of 3 primes: 2+2+11 = 3+5+7 = 5+5+5, and it is the smallest odd number that does.

%p N:= 10^5:

%p P:= select(isprime, [2,seq(i,i=3..N,2)]):

%p nP:=nops(P):

%p V:= Vector(N):

%p for i from 1 to nP do

%p for j from i to nP while P[i]+P[j] <= N do

%p for k from j to nP do

%p n:= P[i]+P[j]+P[k];

%p if n > N then break fi;

%p V[n]:= V[n]+1;

%p od od od:

%p R:= Vector(300):

%p for i from 1 to N by 2 do

%p if V[i] <= 300 and V[i] > 0 and R[V[i]] = 0 then R[V[i]]:= i fi

%p od:

%p convert(R,list);

%Y Cf. A068307, A139321.

%K nonn

%O 0,2

%A _J. M. Bergot_ and _Robert Israel_, Sep 02 2021