A133809 - OEIS

A133809

Numbers that are primally tight, have 2 as first prime and strictly ascending powers.

1, 2, 4, 8, 16, 18, 32, 54, 64, 108, 128, 162, 256, 324, 486, 512, 648, 972, 1024, 1458, 1944, 2048, 2250, 2916, 3888, 4096, 4374, 5832, 8192, 8748, 11250, 11664, 13122, 16384, 17496, 23328, 26244, 32768, 33750, 34992, 39366, 52488, 56250, 65536

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OFFSET

1,2

COMMENTS

All numbers of the form 2^k1*p_2^k2*...*p_n^k_n, where k1 < k2 < ... < k_n and the p_i are the n first primes.

Subset of A073491, A133811 and A133808.

LINKS

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

EXAMPLE

36 = 2^2*3^2 with both exponents being equal is not in the sequence.

PROG

(PARI) isok(n) = {my(f = factor(n)); my(nbf = #f~); if (prod(i=1, nbf, prime(i)) ! = prod(i=1, nbf, f[i, 1]), return (0)); for (j=2, nbf, if (f[j, 2] <= f[j-1, 2], return (0)); ); return (1); } \\ Michel Marcus, Jun 04 2014

(Haskell)

import Data.Set (singleton, deleteFindMin, insert)

a133809 n = a133809_list !! (n-1)

a133809_list = 1 : f (singleton (2, 2, 1)) where

f s = y : f (insert (y*p, p, e+1) $ insert (y*q^(e+1), q, e+1) s')

where q = a151800 p

((y, p, e), s') = deleteFindMin s

-- Reinhard Zumkeller, Apr 14 2015

CROSSREFS

Cf. A025487, A087980, A073491, A133808-A133813.

Cf. A027748, A124010, A151800, A000040, A145108 (subsequence).

Sequence in context: A316900 A076057 A364061 * A128700 A331579 A333225

Adjacent sequences: A133806 A133807 A133808 * A133810 A133811 A133812

KEYWORD

nonn

AUTHOR

Olivier Gérard, Sep 23 2007

STATUS

approved