A134099 Odd nonprimes np such that np-2 is a prime number but np+2 is not.

25, 33, 49, 55, 63, 75, 85, 91, 115, 133, 141, 153, 159, 169, 175, 183, 201, 213, 235, 243, 253, 259, 265, 273, 285, 295, 319, 333, 339, 355, 361, 369, 375, 385, 391, 403, 411, 423, 435, 445, 451, 469, 481, 493, 505, 511, 525, 543, 549, 559, 565, 573, 579

Offset: 1

Author

Enoch Haga, Oct 08 2007

Keywords

• easy
• nonn

Comments

Primes referred to in the example are found in A124582 (see A083370 and compare A124582).

Examples

			a(1) = 25 because it is an odd nonprime preceded by the prime 23 and followed by the odd nonprime 27.
		

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A124582, A083370, A134100, A134101, A007510.

Programs

Mathematica

Select[Range[5,1000,2], !PrimeQ[#] && PrimeQ[#-2] && !PrimeQ[#+2]&] (* Vladimir Joseph Stephan Orlovsky, Feb 03 2012 *)
2#-1&/@(Mean/@SequencePosition[Table[If[PrimeQ[n],1,0],{n,1,601,2}],{1,0,0}]) (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jul 31 2020 *)
Select[Partition[Range[600],5,2],PrimeQ[#[[1]]]&&AllTrue[{#[[3]],#[[5]]},CompositeQ]&][[;;,3]] (* Harvey P. Dale, May 14 2023 *)

UBASIC

10 'primes using counters 20 N=3:print "2 ";:print "3 ";:C=2 30 A=3:S=sqrt(N) 40 B=N\A 50 if B*A=N then 55 55 Q=N+2:R=N-2: if Q<>prmdiv(Q) and N<>prmdiv(N) and R=prmdiv(R) then print Q;N;R;"-";:stop:else N=N+2:goto 30 60 A=A+2 70 if A<=sqrt(N) then 40:stop 81 C=C+1 100 N=N+2:goto 30

Extensions

Definition corrected by Jens Voß, Mar 12 2014

Definition modified by Harvey P. Dale, May 14 2023