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A128693 Numbers of the form 3^k*p, where 1 <= k <= 6 and p is a prime different from 3. 2
6, 15, 18, 21, 33, 39, 45, 51, 54, 57, 63, 69, 87, 93, 99, 111, 117, 123, 129, 135, 141, 153, 159, 162, 171, 177, 183, 189, 201, 207, 213, 219, 237, 249, 261, 267, 279, 291, 297, 303, 309, 321, 327, 333, 339, 351, 369, 381, 387, 393, 405, 411, 417, 423, 447 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Auxiliary sequence for A128694 which gives the number of groups of order a(n).

a(n) is a subset of the composite numbers m having the property that tau(3*m)=tau(m)+2, where tau(m)=A000005(m) (the number of divisors of m). All primes except 3 satisfy this property. - Gary Detlefs, Jan 25 2019

LINKS

Klaus Brockhaus, Table of n, a(n) for n = 1..10000

EXAMPLE

135 = 3^3*5 is a term.

PROG

(MAGMA) [ n: n in [1..450] | #t eq 2 and ((t[1, 1] eq 2 and t[1, 2] eq 1 and t[2, 1] eq 3 and t[2, 2] le 6) or (t[1, 1] eq 3 and t[1, 2] le 6 and t[2, 2] eq 1)) where t is Factorization(n) ];

CROSSREFS

Cf. A128694.

Sequence in context: A099535 A302296 A070999 * A105285 A138922 A044058

Adjacent sequences:  A128690 A128691 A128692 * A128694 A128695 A128696

KEYWORD

nonn

AUTHOR

Klaus Brockhaus, Mar 26 2007

STATUS

approved

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Last modified August 20 18:56 EDT 2019. Contains 326154 sequences. (Running on oeis4.)