A242041 Number of k >= 1 that make n + k! and n - k! both prime.

0, 0, 0, 1, 1, 1, 0, 0, 1, 0, 1, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1, 0, 1, 0, 1, 0, 0, 0, 1, 1, 0, 0, 0, 0, 2, 0, 2, 0, 1, 0, 0, 1, 1, 0, 1, 0, 2, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 2, 0, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 2, 0, 0, 0, 1, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0

Offset: 1

Author

Derek Orr, Aug 12 2014

Keywords

• nonn

Comments

0 <= a(n) <= min{A125162(n), A175940(n)}.

a(n) < A020639(n). - Robert Israel, Aug 12 2014

Links

• Jens Kruse Andersen, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A020639, A125162, A175940.

Programs

Maple

a:= n ->  nops(select(k -> isprime(n+k!) and isprime(n-k!),
      [$1 ..min(numtheory:-factorset(n))-1])):
0, seq(a(n),n=2..100); # Robert Israel, Aug 12 2014

PARI

a(n)=c=0;for(k=1,n,if(ispseudoprime(n+k!)&&ispseudoprime(n-k!),c++));return(c)
vector(100,n,a(n))