A159560 Minimal recursive sequence beginning with 3 such that A001221(a(n)) = A001221(n).

3, 4, 5, 7, 10, 11, 13, 16, 18, 19, 20, 23, 24, 26, 27, 29, 33, 37, 38, 39, 40, 41, 44, 47, 48, 49, 50, 53, 60, 61, 64, 65, 68, 69, 72, 73, 74, 75, 76, 79, 84, 89, 91, 92, 93, 97, 98, 101, 104, 106, 108, 109, 111, 112, 115, 116, 117, 121, 126, 127, 129, 133, 137, 141, 150

Offset: 2

Author

Vladimir Shevelev, Apr 15 2009

Keywords

• nonn

Links

• Amiram Eldar, Table of n, a(n) for n = 2..10000
• V. Shevelev, Several results on sequences which are similar to the positive integers, arXiv:0904.2101 [math.NT], 2009.

Crossrefs

Cf. A001221, A159559.

Programs

Maple

A001221 := proc(n) nops(numtheory[factorset](n)) ; end proc:
A159560 := proc(n) option remember; if n = 2 then 3; else for a from procname(n-1)+1 do if A001221(a) = A001221(n) then return a; end if; end do: end if; end proc:
seq(A159560(n),n=2..70) ; # R. J. Mathar, Oct 30 2010

Mathematica

a[2] = 3; a[n_] := a[n] = For[an = a[n - 1] + 1, True, an++, If[PrimeNu[an] == PrimeNu[n], Return[an]]];
Table[a[n], {n, 2, 66}] (* Jean-François Alcover, Nov 14 2017 *)

Formula

a(n+1) = min{m > a(n): A001221(m) = A001221(n+1)}.

Extensions

Corrected (4 inserted, 8 removed) and extended beyond 53 by R. J. Mathar, Oct 30 2010