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 A124936 Numbers k such that k - 1 and k + 1 are semiprimes. 18
 5, 34, 50, 56, 86, 92, 94, 120, 122, 142, 144, 160, 184, 186, 202, 204, 214, 216, 218, 220, 236, 248, 266, 288, 290, 300, 302, 304, 320, 322, 328, 340, 392, 394, 412, 414, 416, 446, 452, 470, 472, 516, 518, 528, 534, 536, 544, 552, 580, 582, 590, 634, 668 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All but the first term are even. LINKS Zak Seidov, Table of n, a(n) for n = 1..1000 FORMULA a(n) = A092207(n) + 1; at n>=2, a(n) = 2*A082130(n-1). MATHEMATICA lst={}; Do[If[Plus@@Last/@FactorInteger[n-1]==2&&Plus@@Last/@FactorInteger[n+1]==2, AppendTo[lst, n]], {n, 7!}]; lst (* Vladimir Joseph Stephan Orlovsky, Feb 01 2009 *) Select[Range[2, 700], PrimeOmega[# + 1] == PrimeOmega[# - 1] == 2 &] (* Vincenzo Librandi, Mar 30 2015 *) PROG (MAGMA) IsSemiprime:=func< n | &+[k[2]: k in Factorization(n)] eq 2 >; [ n: n in [1..700] | IsSemiprime(n+1) and IsSemiprime(n-1)]; // Vincenzo Librandi, Mar 30 2015 (PARI) list(lim)=if(lim<5, return([])); my(v=List([5]), x=1, y=1); forfactored(z=7, lim\1+1, if(vecsum(z[2][, 2])==2 && vecsum(x[2][, 2])==2, listput(v, z[1]-1)); x=y; y=z); Vec(v) \\ Charles R Greathouse IV, May 22 2018 CROSSREFS Cf. A092207 (Numbers k such that k and k+2 are semiprimes), A086005 (Numbers n such that k, k -+ 1 are semiprimes), A086006 (Primes p such that 2*p-1 and 2*p+1 are semiprimes), A082130 (2*k-1 and 2*k+1 are semiprimes). Sequence in context: A193325 A303693 A256373 * A213063 A268281 A223137 Adjacent sequences:  A124933 A124934 A124935 * A124937 A124938 A124939 KEYWORD nonn AUTHOR Zak Seidov, Nov 13 2006 STATUS approved

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Last modified January 20 19:53 EST 2020. Contains 331096 sequences. (Running on oeis4.)