A137462 a(n) + a(n-1) = n-th semiprime.

1, 3, 3, 6, 4, 10, 5, 16, 6, 19, 7, 26, 8, 27, 11, 28, 18, 31, 20, 35, 22, 36, 26, 39, 30, 44, 33, 49, 36, 50, 37, 54, 39, 55, 40, 66, 45, 70, 48, 71, 50, 72, 51, 78, 55, 79, 62, 80, 63, 82, 64, 91, 67, 92, 69, 97, 72, 105, 73, 110, 75, 112, 82, 119, 83, 120, 85, 121, 88, 125

Offset: 0

Author

Jonathan Vos Post, Apr 19 2008

Keywords

• easy
• nonn

Comments

This is to A001358 as A036467 is to A000040.

Links

• Harvey P. Dale, Table of n, a(n) for n = 0..1000

Crossrefs

Cf. A001358, A036467, A065516.

Programs

Maple

A001358 := proc(n) option remember ; if n =1 then 4; else for a from A001358(n-1)+1 do if numtheory[bigomega](a) = 2 then RETURN(a) ; fi ; od: fi ; end: A137462 := proc(n) option remember; if n =0 then 1; else A001358(n)-A137462(n-1) ; fi ; end: seq(A137462(n),n=0..100) ; # R. J. Mathar, Apr 23 2008

Mathematica

Module[{nn=300,sp,k=1},sp=Select[Range[nn],PrimeOmega[#]==2&];Join[{1}, Table[k=sp[[n]]-k,{n,Length[sp]}]]] (* Harvey P. Dale, Apr 30 2015 *)

Formula

a(n) + a(n-1) = A001358(n).

Extensions

More terms from R. J. Mathar, Apr 23 2008