A097523 a(n) = least k such that k - prime(n) and k + prime(n) are both prime.

5, 8, 8, 10, 18, 16, 20, 22, 30, 32, 36, 42, 48, 46, 50, 56, 72, 66, 70, 78, 76, 84, 90, 92, 100, 132, 108, 120, 114, 116, 130, 138, 140, 142, 162, 156, 160, 168, 170, 176, 210, 186, 198, 196, 200, 202, 222, 226, 230, 232, 246, 252, 246, 258, 264, 294, 272, 276

Offset: 1

Author

Pierre CAMI, Aug 27 2004

Keywords

• easy
• nonn

Examples

			Prime(10) = 29; both 32 - 29 = 3 and 32 + 29 = 61 are prime, and 32 is the smallest integer for which this is the case, so a(10) = 32.
		

Links

• Pierre CAMI, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000040, A087711.

Programs

Maple

f:= proc(n) local p, k;
  p:= ithprime(n);
  for k from p+1 by 2 do
    if isprime(k+p) and isprime(k-p) then return k fi
  od
end proc:
f(1):= 5:
map(f, [$1..100]); # Robert Israel, Jul 26 2015

Mathematica

f[n_] := Block[{k = Prime[n], p = Prime[n]}, While[ !PrimeQ[k - p] || !PrimeQ[k + p], k++ ]; k]; Table[ f[n], {n, 60}] (* Robert G. Wilson v, Aug 28 2004 *)

Formula

a(n) = A087711(A000040(n)). - Robert Israel, Jul 26 2015

Extensions

Corrected by Robert G. Wilson v, Aug 28 2004