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

1,4

Links

Table of n, a(n) for n=1..56.

Example

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

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

Crossrefs

Cf. A001097, A164276.

Sequence in context: A354114 A175220 A272479 * A227437 A352314 A363328

Adjacent sequences: A167424 A167425 A167426 * A167428 A167429 A167430

Keyword

nonn

Author

Juri-Stepan Gerasimov, Nov 03 2009

Extensions

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

Status

approved