A156874 -id:A156874 - cp's OEIS frontend

A305894 Filter sequence for a(Sophie Germain primes) = constant sequences.

1, 2, 2, 3, 2, 4, 5, 6, 7, 8, 2, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 2, 20, 21, 22, 23, 24, 2, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 2, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 2, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 2, 76, 77, 78, 79, 80, 2, 81

Offset: 1

Antti Karttunen, Jul 01 2018

nonn

For all i, j:
  a(i) = a(j) => A305800(i) = A305800(j),
  a(i) = a(j) => A305978(i) = A305978(j).

Antti Karttunen, Table of n, a(n) for n = 1..100000

Cf. A005384, A156660, A156874, A305800, A305810, A305978.

up_to = 100000;
A156660(n) = (isprime(n)&&isprime(2*n+1)); \\ From A156660
partialsums(f,up_to) = { my(v = vector(up_to), s=0); for(i=1,up_to,s += f(i); v[i] = s); (v); }
v156874 = partialsums(A156660, up_to);
A156874(n) = v156874[n];
A305894(n) = if(n<2,n,if(A156660(n),2,1+n-A156874(n)));

a(1) = 1; for n > 1, a(n) = 2 if A156660(n) == 1 [when n is in A005384 = 2, 3, 5, 11, 23, 29, 41, 53, 83, 89, 113, ...], otherwise a(n) = 1+n-A156874(n).

cp's OEIS Frontend

A305894 Filter sequence for a(Sophie Germain primes) = constant sequences.

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