A163468 Indices k such that half of the k-th nonprime nonnegative integer is prime.
3, 4, 7, 9, 15, 18, 24, 27, 33, 43, 45, 54, 61, 64, 71, 80, 89, 93, 103, 109, 113, 122, 129, 139, 151, 157, 161, 168, 172, 179, 201, 208, 217, 220, 237, 241, 250, 261, 268, 279, 288, 291, 308, 311, 318, 321, 341, 361, 368, 371, 377, 388, 391, 408, 418, 428, 440
Offset: 1
Examples
3 is a term because the 3rd nonprime nonnegative integer is 4, and 4/2 = 2 is prime. 4 is a term because the 4th nonprime nonnegative integer is 6, and 6/2 = 3 is prime. 7 is a term because the 7th nonprime nonnegative integer is 10, and 10/2 = 5 is prime.
Links
- Harvey P. Dale, Table of n, a(n) for n = 1..1000
Programs
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Maple
A141468 := proc(n) option remember ; if n = 1 then 0; else for a from procname(n-1)+1 do if not isprime(a) then RETURN(a) ; fi; od: fi; end: for n from 1 to 600 do npr := A141468(n) ; if type(npr,'even') then if isprime(npr/2) then printf("%d,",n) ; fi; fi; od: # R. J. Mathar, Aug 01 2009
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Mathematica
Flatten[Position[Select[Range[600],!PrimeQ[#]&],?(PrimeQ[#/2]&)]]+1 (* _Harvey P. Dale, May 05 2016 *)
Formula
a(n) = 2 + A065897(n). - R. J. Mathar, Aug 01 2009
Extensions
a(8) corrected by R. J. Mathar, Aug 01 2009
Edited by Jon E. Schoenfield, Mar 04 2019
Comments