|
%I #12 Jul 22 2019 19:22:09
%S 1,2,1,4,0,6,3,2,2,8,1,12,2,2,4,16,2,10,5,2,8,14,2,14,2,8,9,16,0,20,
%T 11,4,4,14,2,18,16,8,7,28,2,32,6,6,10,24,5,14,14,8,21,42,0,22,8,14,12,
%U 22,4,24,18,14,20,14,2,44,14,14,10,34,5,46,20,4,18,38,4,38,15,10,16,46,2
%N Number of k, 1 <= k <= n, where {k (n+1-k) + 1} is prime.
%e For n = 7, 2*6 + 1, 4*4 + 1 and 6*2 + 1 are prime, so a(7) = 3.
%p A057458 := proc(n)
%p a := 0 ;
%p for k from 1 to n do
%p if isprime(1+k*(n+1-k)) then
%p a := a+1 ;
%p end if;
%p end do:
%p a;
%p end proc:
%p seq(A057458(n),n=1..80) ; # _R. J. Mathar_, Jan 28 2014
%t Table[Count[Table[k(n+1-k)+1,{k,n}],_?PrimeQ],{n,90}] (* _Harvey P. Dale_, Jul 22 2019 *)
%K nonn
%O 1,2
%A _Leroy Quet_, Sep 26 2000
|
