A163975 n-th nonprime - (-1)^(n-th nonprime).

-1, 2, 3, 5, 7, 10, 9, 11, 13, 16, 15, 17, 19, 22, 21, 23, 26, 25, 28, 27, 29, 31, 34, 33, 36, 35, 37, 40, 39, 41, 43, 46, 45, 47, 50, 49, 52, 51, 53, 56, 55, 58, 57, 59, 61, 64, 63, 66, 65, 67, 70, 69, 71, 73, 76, 75, 78, 77, 79, 82, 81, 83, 86, 85, 88, 87, 89, 92, 91, 94, 93

Offset: 1

Author

Juri-Stepan Gerasimov, Aug 07 2009

Keywords

• sign
• easy
• less

Examples

			a(1)=0-(-1)^0=-1. a(2)=1-(-1)^1=2.
		

Links

• G. C. Greubel, Table of n, a(n) for n = 1..5000

Crossrefs

Cf. A141468.

Programs

Maple

A103889 := proc(n) n-(-1)^n ; end:
A141468 := proc(n) option remember ; if n <= 2 then n-1 ; else for a from procname(n-1)+1 do if not isprime(a) then RETURN(a); fi; od: fi; end:
A163975 := proc(n) A103889(A141468(n)) ; end: seq( A163975(n),n=1..120) ;

Mathematica

nonPrime[n_Integer] := FixedPoint[n + PrimePi@# &, n + PrimePi@n]; Table[nonPrime[n] - (-1)^(nonPrime[n]), {n, 0, 50}] (* G. C. Greubel, Aug 24 2017 *)

PARI

for(n=1,1e3,if(!isprime(n), print1(n - (-1)^n", "))) \\ Charles R Greathouse IV, Jun 10 2015

Formula

a(n) = A141468(n) - (-1)^A141468(n).

a(n) = A103889(A141468(n)), n>0.

Extensions

Entries checked by R. J. Mathar, Aug 29 2009