A393829 a(n) is the greatest odd semiprime < prime(n).

9, 9, 15, 15, 21, 25, 25, 35, 39, 39, 39, 51, 57, 57, 65, 69, 69, 77, 77, 87, 95, 95, 95, 95, 95, 111, 123, 129, 133, 133, 145, 145, 155, 161, 161, 169, 177, 177, 187, 187, 187, 187, 209, 221, 221, 221, 221, 237, 237, 249, 253, 259, 267, 267, 267, 267, 267, 291

Offset

5,1

Links

James C. McMahon, Table of n, a(n) for n = 5..10000

Mathematica

a[n_]:=Module[{c=Prime[n]-2}, While[PrimeOmega[c]!=2, c=c-2]; c]; Array[a, 58, 5] (* James C. McMahon, Mar 07 2026 *)

Prog

(PARI) a393829(n) = forstep(k=prime(n)-2, 9, -2, if(bigomega(k)==2, return(k))); -1

Crossrefs

Cf. A046315, A393819, A393828.

Sequence in context: A124475 A179057 A350919 * A144418 A393818 A394445

Adjacent sequences: A393826 A393827 A393828 * A393830 A393831 A393832

Keyword

nonn,easy

Author

Hugo Pfoertner, Mar 07 2026

Status

approved