The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A243428 Semiprimes A001358(n) such that A001358(n) + 2^n is also a semiprime. 1
 4, 6, 10, 14, 22, 25, 35, 39, 95, 123, 129, 177, 289, 309, 327, 355, 415, 445, 471, 497, 543, 689 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Generated by n = 1, 2, 4, 5, 8, 9, 13, 15, 34, 42, 43, 57, 90, 99, 105, 112, 131, 136, 145, 153, 170, 184, ... LINKS EXAMPLE 4 is in this sequence because A001358(1) + 2^1 = 6 is also semiprime. MATHEMATICA NextSemiPrime[n_, k_: 1] := Block[{c = 0, sgn = Sign[k]}, sp = n + sgn; While[c < Abs[k], While[ PrimeOmega[sp] != 2, If[sgn < 0, sp--, sp++]]; If[ sgn < 0, sp--, sp++]; c++]; sp + If[sgn < 0, 1, -1]]; SemiPrimePi[n_] := Sum[ PrimePi[ n/Prime[i]] - i + 1, {i, PrimePi[ Sqrt[ n]] }]; sp = 4; lst = {}; While[ sp < 1001, If[ PrimeOmega[sp + 2^SemiPrimePi@ sp] == 2, AppendTo[lst, sp]; Print@ sp]; sp = NextSemiPrime@ sp; c++]; lst (* Robert G. Wilson v, Jun 20 2014 *) PROG (PARI) list(lim)=my(v=List(), t); forprime(p=2, sqrt(lim), t=p; forprime(q=p, lim\t, listput(v, t*q))); vecsort(Vec(v)) \\ From A001358 sp=list(700); s=[]; for(n=1, #sp, if(bigomega(sp[n]+2^n)==2, s=concat(s, sp[n]))); s \\ Colin Barker, Jun 05 2014 CROSSREFS Cf. A001358. Sequence in context: A325042 A137860 A184335 * A091376 A100484 A076924 Adjacent sequences:  A243425 A243426 A243427 * A243429 A243430 A243431 KEYWORD nonn,less AUTHOR Juri-Stepan Gerasimov, Jun 05 2014 EXTENSIONS One term inserted, and more terms from Colin Barker, Jun 05 2014 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified June 30 09:10 EDT 2022. Contains 354920 sequences. (Running on oeis4.)