A393819 a(n) is the greatest odd semiprime < 2*prime(n).

9, 9, 21, 25, 33, 35, 39, 57, 57, 69, 77, 85, 93, 95, 115, 121, 133, 141, 145, 155, 161, 177, 187, 201, 205, 213, 217, 221, 253, 259, 267, 267, 295, 301, 309, 323, 329, 341, 355, 361, 381, 381, 393, 395, 417, 445, 453, 453, 453, 473, 481, 501, 511, 519, 537, 537

Offset

3,1

Links

James C. McMahon, Table of n, a(n) for n = 3..10000

Maple

a:= proc(n) local k; for k from 2*ithprime(n)-1 by -2
      while numtheory[bigomega](k)<>2 do od; k
    end:
seq(a(n), n=3..58);  # Alois P. Heinz, Mar 06 2026

Mathematica

a[n_]:=Module[{c=2Prime[n]-1}, While[PrimeOmega[c]!=2, c=c-2]; c]; Array[a, 56, 3] (* James C. McMahon, Mar 06 2026 *)

Prog

(PARI) a393819(n) = forstep(k=2*prime(n)-1, 9, -2, if(bigomega(k)==2, return(k))); -1

Crossrefs

Cf. A001747, A046315, A393818.

Sequence in context: A161365 A214831 A197497 * A053456 A144424 A383079

Adjacent sequences: A393816 A393817 A393818 * A393820 A393821 A393822

Keyword

nonn,easy

Author

Hugo Pfoertner, Mar 06 2026

Status

approved