A175191 a(n) = the smallest positive integer such that (the n-th prime)+2*a(n) is composite.

1, 3, 2, 1, 2, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 1, 1, 1, 2, 1, 2, 1, 1, 1, 1, 2, 1, 2, 1, 1, 1, 1, 1, 2, 1, 2, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 2, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2, 1

Offset: 1

Author

Leroy Quet, Mar 01 2010

Keywords

• nonn

Comments

Every term except a(2) (which is 3) is either 1 or 2.

Links

• G. C. Greubel, Table of n, a(n) for n = 1..1000

Programs

Maple

a:= n-> `if`(n=2, 3, `if`(isprime(ithprime(n)+2), 2, 1)):
seq(a(n), n=1..105);  # Alois P. Heinz, Oct 02 2020

Mathematica

spi[n_]:=Module[{i=1},While[PrimeQ[Prime[n]+2i],i++];i]; Array[spi,110] (* Harvey P. Dale, Apr 23 2012 *)

Extensions

Extended by Ray Chandler, Mar 04 2010