

A109027


Numbers that have exactly seven prime factors counted with multiplicity (A046308) whose digit reversal is different and also has 7 prime factors (with multiplicity).


10



8820, 21240, 21708, 21780, 21920, 23280, 23472, 23625, 23800, 25560, 25584, 25758, 26280, 27432, 27504, 27888, 27900, 28836, 29250, 29403, 29736, 29970, 30492, 34884, 36828, 40338, 40572, 40950, 41976, 42228, 42984, 43659, 43956, 44128
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OFFSET

1,1


COMMENTS

This sequence is the k = 7 instance of the series which begins with k = 1 (emirps), k = 2, k = 3, k = 4, k = 5 (A109025), k = 6 (A109026).


LINKS

Eric Weisstein's World of Mathematics, Emirp.
Eric Weisstein and Jonathan Vos Post, Emirpimes.


EXAMPLE

a(20) = 29403 is in this sequence because 29403 = 3^5 * 11^2 has exactly 7 prime factors counted with multiplicity and reverse(29403) = 30492 = 2^2 * 3^2 * 7 * 11^2 also has exactly 7 prime factors counted with multiplicity.


MATHEMATICA

Select[Range[45000], !PalindromeQ[#]&&PrimeOmega[#]==PrimeOmega[ IntegerReverse[ #]] ==7&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, May 02 2019 *)


PROG

(PARI) is(n) = {
my(r = fromdigits(Vecrev(digits(n))));
n!=r && bigomega(n) == 7 && bigomega(r) == 7


CROSSREFS

Cf. A046308, A006567, A097393, A109018, A109023, A109024, A109025, A109026, A109028, A109029, A109030, A109031.


KEYWORD

nonn,base


AUTHOR



STATUS

approved



