A328903 - OEIS

%I #25 Nov 01 2019 11:43:05

%S 0,0,1,0,2,2,0,2,3,0,3,4,0,3,5,0,4,2,0,6,5,0,4,5,0,6,7,0,6,9,0,6,8,0,

%T 9,8,0,7,7,0,9,10,0,11,2,0,12,12,0,10,11,0,11,14,0,3,10,0,10,12,0,16,

%U 12,0,16,14,0,14,19,0,13,17,0,12,16,0,15,2,0

%N Number of primes that are a concatenation of two positive integers whose sum is n.

%C First n > 1 with n != 0 (mod 3) and a(n) = 0 is n = 4477. - _Alois P. Heinz_, Oct 30 2019

%H Alois P. Heinz, <a href="/A328903/b328903.txt">Table of n, a(n) for n = 0..20000</a>

%F a(n) = 0 if n = 1 or n == 0 (mod 3). - _Alois P. Heinz_, Oct 30 2019

%e 1(-), 2(11), 3(-), 4(13, 31), 5(23, 41), 6(-), 7(43, 61), 8(17, 53, 71), 9(-), 10(19, 37, 73), 11(29, 47, 83, 101), 12(-), 13(67, 103, 211), ...

%p a:= proc(n) option remember; `if`(irem(n, 3)=0, 0, add(

%p      `if`(isprime(parse(cat(i, n-i))), 1, 0), i=1..n-1))

%p     end:

%p seq(a(n), n=0..100);  # _Alois P. Heinz_, Oct 30 2019

%Y Cf. A000040, A008486, A008585, A066686, A067574, A154530, A328932.

%K nonn,base,look,easy

%O 0,5

%A _Juri-Stepan Gerasimov_, Oct 30 2019

%E More terms from _Alois P. Heinz_, Oct 30 2019