OFFSET
1,2
FORMULA
Hardy and Littlewood conjectured that : a(n) ~ c* sqrt(n)/Log(n) where c = prod(p prime, 1 - (-1)^((p-1)/2)/(p-1) ) = 1, 3727...
MATHEMATICA
Accumulate[Table[If[PrimeQ[k^2+1], 1, 0], {k, 80}]] (* Harvey P. Dale, Jan 08 2020 *)
PROG
(PARI) for(n=1, 200, print1(sum(i=1, n, if(isprime(i^2+1)-1, 0, 1)), ", "))
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Benoit Cloitre, Jun 09 2002
STATUS
approved