A393829 - OEIS

%I #8 Mar 07 2026 13:55:11

%S 9,9,15,15,21,25,25,35,39,39,39,51,57,57,65,69,69,77,77,87,95,95,95,

%T 95,95,111,123,129,133,133,145,145,155,161,161,169,177,177,187,187,

%U 187,187,209,221,221,221,221,237,237,249,253,259,267,267,267,267,267,291

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

%H James C. McMahon, <a href="/A393829/b393829.txt">Table of n, a(n) for n = 5..10000</a>

%t 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 *)

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

%Y Cf. A046315, A393819, A393828.

%K nonn,easy

%O 5,1

%A _Hugo Pfoertner_, Mar 07 2026