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

1,1

Comments

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

Links

Table of n, a(n) for n=1..45.

Formula

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

Example

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

Mathematica

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

Prog

(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

Crossrefs

Cf. A000961, A034699.

Sequence in context: A184432 A003274 A259470 * A078878 A095361 A095362

Adjacent sequences: A121312 A121313 A121314 * A121316 A121317 A121318

Keyword

nonn

Author

Paul Richards, Aug 28 2006

Extensions

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

Status

approved