A167427 Largest non-isolated nonprime < n-th non-isolated (or twin) prime.

1, 1, 1, 10, 10, 16, 16, 28, 28, 40, 40, 58, 58, 70, 70, 100, 100, 106, 106, 136, 136, 148, 148, 178, 178, 190, 190, 196, 196, 226, 226, 238, 238, 268, 268, 280, 280, 310, 310, 346, 346, 418, 418, 430, 430, 460, 460, 520, 520, 568, 568, 598, 598, 616, 616, 640

Offset: 1

Author

Juri-Stepan Gerasimov, Nov 03 2009

Keywords

• nonn

Examples

			a(1)=1 (<3); a(2)=1 (<5); a(3)=1 (<7); a(4)=10 (<11).
		

Crossrefs

Cf. A001097, A164276.

Programs

Maple

isA164276 := proc(n) not isprime(n) and ( not isprime(n+1) or not isprime(n-1) ) ; end proc:
isA001097 := proc(n) isprime(n) and ( isprime(n+2) or isprime(n-2) ); end proc:
A001097 := proc(n) option remember; if n =1 then 3; else for a from procname(n-1)+2 by 2 do if isA001097(a) then return a; end if; end do: end if; end proc:
A167427 := proc(n) for a from A001097(n)-1 by -1 do if isA164276(a) then return a; end if; end do: end proc:
seq(A167427(n),n=1..120) ; # R. J. Mathar, May 30 2010

Extensions

Entries checked by R. J. Mathar, May 30 2010