A109403 Examine the sequence of all (even or odd) semiprimes, A001358, and record the averages of any pair of successive terms of the same parity.

5, 12, 18, 50, 53, 56, 60, 67, 89, 92, 113, 120, 126, 131, 144, 160, 173, 184, 186, 204, 211, 216, 220, 236, 242, 248, 251, 266, 276, 288, 290, 293, 300, 304, 307, 320, 322, 328, 337, 340, 368, 374, 379, 384, 392

Offset

1,1

Links

N. J. A. Sloane, Table of n, a(n) for n = 1..20000

Example

5 is OK because sp(1)=4, sp(2)=6 and (4+6)/2=5; 12 is OK because sp(4)=10, sp(5)=14 and (10+14)/2=12; sp(n)=n-th semiprime.

Maple

A001358 := proc(n) option remember; local a; if n = 1 then 4; else for a from procname(n-1)+1 do if numtheory[bigomega](a) = 2 then return a; end if; end do: end if; end proc:
# To produce a b-file
M:=50000; c:=0;
for n from 1 to M do
if type(A001358(n)+A001358(n+1), even) then c:=c+1;
m:=(A001358(n)+A001358(n+1))/2;
lprint(c, m);
fi; od; # N. J. A. Sloane, Aug 27 2020

Mathematica

 Select[Mean/@Partition[Select[Range[400], PrimeOmega[#]==2&], 2, 1], IntegerQ] (* Harvey P. Dale, Aug 27 2020 *)

Crossrefs

Cf. A001358, A100484.

Sequence in context: A294143 A101757 A174518 * A093084 A063614 A048928

Adjacent sequences: A109400 A109401 A109402 * A109404 A109405 A109406

Keyword

nonn

Author

Zak Seidov, Jun 27 2005

Extensions

Definition corrected by N. J. A. Sloane, Aug 26 2020. The original definition appeared to be based on A100484, but that was incorrect. Thanks to Harvey P. Dale for pointing out that something was wrong.

Status

approved