A096870 Difference between the n-th repunit and the previous prime.

4, 2, 2, 18, 2, 20, 10, 2, 14, 52, 8, 18, 58, 58, 72, 4, 20, 74, 84, 58, 14, 18, 82, 168, 28, 50, 168, 84, 8, 138, 112, 82, 2, 28, 2, 62, 34, 50, 74, 24, 8, 54, 204, 22, 428, 40, 118, 200, 72, 40, 30, 42, 284, 44, 114, 268, 80, 18, 4, 74, 142, 182, 140, 112, 214, 152, 90

Offset: 2

Author

Robert G. Wilson v, Jul 12 2004

Keywords

• nonn

Links

• Robert Israel, Table of n, a(n) for n = 2..1000

Crossrefs

Cf. A096869.

Programs

Maple

f:= proc(n) local r; r:= (10^n-1)/9; r - prevprime(r) end proc:
map(f, [$2..100]); # Robert Israel, Feb 23 2017

Mathematica

PrevPrim[n_] := Block[{k = n - 1}, While[ !PrimeQ[k], k-- ]; k]; Table[(10^n - 1)/9 - PrevPrim[(10^n - 1)/9], {n, 2, 70}]
Table[With[{ru=(10^n-1)/9},ru-NextPrime[ru,-1]],{n,2,70}] (* Harvey P. Dale, Aug 14 2011 *)

Formula

a(n) = A002275(n) - A096498(n) = A096499(n) - A096869(n).