A142352 Primes of the form n/2*(c(n)-r(n)), where c(n)=n-th composite and r(n)=n-th nonprime.

2, 5, 7, 11, 17, 23, 19, 31, 23, 43, 59, 31, 67, 47, 71, 73, 79, 83, 59, 89, 53, 107, 109, 113, 127, 131, 89, 137, 139, 97, 149, 103, 157, 163, 167, 173, 179, 181, 127, 191, 131, 197, 199, 223, 151, 227, 22, 9, 233, 239, 241, 163, 263, 271, 181, 277, 281, 283, 293

Offset

1,1

Links

Table of n, a(n) for n=1..59.

Example

If n=12, then 12/2*(c(12)-r(12))=6/(21-18)=2=a(1).
If n=30, then 30/2*(c(30)-r(30))=15/(45-42)=5=a(2).
If n=42, then 42/2*(c(42)-r(42))=21/(60-57)=7=a(3).
If n=66, then 66/2*(c(66)-r(66))=33/(91-88)=11=a(4).
If n=68, then 68/2*(c(68)-r(68))=34/(93-91)=17=a(5), etc.

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: A002808 := proc(n) option remember ; A141468(n+2) ; end: for n from 1 to 3000 do p := n/(A002808(n)-A141468(n))/2 ; if type(p, 'integer') then if isprime(p) then printf("%d, ", p) ; fi; fi; od: # R. J. Mathar, Jan 23 2009

Crossrefs

Cf. A002808, A141468.

Sequence in context: A023213 A162575 A066408 * A290012 A062044 A077128

Adjacent sequences: A142349 A142350 A142351 * A142353 A142354 A142355

Keyword

nonn

Author

Juri-Stepan Gerasimov, Sep 21 2008

Extensions

Corrected and extended by R. J. Mathar, Jan 23 2009

Status

approved