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A061673 Even numbers k such that k+1 and k-1 are both composite. 3
26, 34, 50, 56, 64, 76, 86, 92, 94, 116, 118, 120, 122, 124, 134, 142, 144, 146, 154, 160, 170, 176, 184, 186, 188, 202, 204, 206, 208, 214, 216, 218, 220, 236, 244, 246, 248, 254, 260, 266, 274, 286, 288, 290, 296, 298, 300, 302, 304, 320, 322, 324, 326 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000

EXAMPLE

a(3)=50 because 50 - 1 = 49 and 50 + 1 = 51 and both 49 and 51 are composite.

MATHEMATICA

fQ[n_] := !PrimeQ[n - 1] && !PrimeQ[n + 1]; Select[2 Range@ 163, fQ]

Select[Range[2, 400, 2], AllTrue[#+{1, -1}, CompositeQ]&] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, Sep 01 2014 *)

PROG

(PARI) { n=0; forstep (a=2, 3986, 2, if (!isprime(a+1) && !isprime(a-1), write("b061673.txt", n++, " ", a)) ) } \\ Harry J. Smith, Jul 26 2009

(Haskell)

a061673 n = a061673_list !! (n-1)

a061673_list = filter bothComp [4, 6..] where

   bothComp n = (1 - a010051 (n-1)) * (1 - a010051 (n+1)) > 0

-- Reinhard Zumkeller, Feb 27 2011

(GAP) Filtered([0, 2..340], n->not IsPrime(n-1) and not IsPrime(n+1)); # Muniru A Asiru, Jul 01 2018;

CROSSREFS

Cf. A055670.

A025583(n-1) - 1.

Sequence in context: A100393 A133635 A167705 * A072571 A307033 A058763

Adjacent sequences:  A061670 A061671 A061672 * A061674 A061675 A061676

KEYWORD

easy,nonn,nice

AUTHOR

Enoch Haga, Jun 16 2001

STATUS

approved

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Last modified November 22 16:13 EST 2019. Contains 329396 sequences. (Running on oeis4.)