A127019 a(n) is the least semiprime s such that s - 2*n is also a semiprime.

6, 10, 10, 14, 14, 21, 35, 22, 22, 26, 26, 33, 35, 34, 34, 38, 38, 46, 77, 46, 46, 58, 55, 57, 65, 58, 58, 62, 62, 69, 77, 74, 87, 74, 74, 82, 95, 82, 82, 86, 86, 93, 95, 94, 94, 106, 115, 106, 119, 106, 106, 118, 115, 118, 119, 118, 118, 122, 122, 129, 143, 133, 141, 134

Offset: 1

Author

Rick L. Shepherd, Jan 02 2007

Keywords

• nonn

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A127018, A020484.

Programs

Maple

N:= 500: # for terms before the first term > N
P:= select(isprime, [2,seq(i,i=3..N/2,2)]):
SP:= select(`<=`,{seq(seq(P[i]*P[j],i=1..j),j=1..nops(P))},N):
R:= NULL:
for n from 1 do
  Q:= SP intersect (SP +~ (2*n));
  if Q = {} then break fi;
  R:= R, min(Q)
od:
R; # Robert Israel, Jan 30 2025

Mathematica

sp=Select[Range[1000],PrimeOmega[#]==2&];seq={};Do[s=0;Until[t=sp[[s]]-2n;PrimeOmega[t]==2&&t>0,s++];AppendTo[seq,sp[[s]]],{n,64}];seq (* James C. McMahon, Dec 30 2024 *)