A081218 Greatest squarefree number not exceeding n-th prime power which is not prime.

1, 3, 7, 7, 15, 23, 26, 31, 47, 62, 79, 119, 123, 127, 167, 241, 255, 287, 341, 359, 511, 527, 623, 727, 839, 959, 1023, 1330, 1367, 1679, 1847, 2047, 2186, 2195, 2207, 2399, 2807, 3122, 3478, 3719, 4094, 4487, 4911, 5039, 5327, 6239, 6559, 6857, 6887, 7919

Offset: 1

Author

Reinhard Zumkeller, Mar 10 2003

Keywords

• nonn

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A025475, A070321, A081217.

Programs

Mathematica

f[n_] := Module[{k = n-1}, While[!SquareFreeQ[k], k--]; k]; Join[{1}, f /@ Select[Range[8000], CompositeQ[#] && PrimePowerQ[#] &]] (* Amiram Eldar, Mar 26 2025 *)

Formula

a(n) = A070321(A025475(n)).