A121315 Products of two consecutive prime powers.

2, 6, 12, 20, 35, 56, 72, 99, 143, 208, 272, 323, 437, 575, 675, 783, 899, 992, 1184, 1517, 1763, 2021, 2303, 2597, 3127, 3599, 3904, 4288, 4757, 5183, 5767, 6399, 6723, 7387, 8633, 9797, 10403, 11021, 11663, 12317, 13673, 15125, 15875, 16256, 16768

Offset: 1

Author

Paul Richards, Aug 28 2006

Keywords

• nonn

Comments

For some algorithms for finding A034699(n), the numbers in this sequence represent a worst-case scenario of execution time.

Examples

			437 = 19*23 and none of the intervening integers (20,21,22) are prime powers.
		

Crossrefs

Cf. A000961, A034699.

Programs

Mathematica

t = Join[{1}, Select[Range[2, 131], Mod[ #, # - EulerPhi[ # ]] == 0 &]]; Most@t*Rest@t (* Robert G. Wilson v, Sep 02 2006 *)

PARI

lista(nn) = v = concat(1, select(x->isprimepower(x), vector(nn, n, n))); for (n=1, #v-1, print1(v[n]*v[n+1], ", ")); \\ Michel Marcus, Mar 20 2015

Formula

a(n) = A000961(n)*A000961(n+1).

Extensions

More terms from Robert G. Wilson v, Sep 02 2006