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A109024 Numbers that have exactly four prime factors counted with multiplicity (A014613) whose digit reversal is different and also has 4 prime factors (with multiplicity).

%I A109024 #34 Feb 16 2025 08:32:58
%S A109024 126,225,294,315,459,488,492,513,522,558,621,650,738,837,855,884,954,
%T A109024 1035,1062,1098,1107,1197,1206,1236,1287,1305,1422,1518,1617,1665,
%U A109024 1917,1926,1956,1962,1989,2004,2034,2046,2068,2104,2148,2170,2180,2223,2226
%N A109024 Numbers that have exactly four prime factors counted with multiplicity (A014613) whose digit reversal is different and also has 4 prime factors (with multiplicity).
%C A109024 This sequence is the k = 4 instance of the series which begins with k = 1, k = 2, k = 3 (A109023).
%H A109024 David A. Corneth, <a href="/A109024/b109024.txt">Table of n, a(n) for n = 1..10000</a>
%H A109024 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/AlmostPrime.html">Almost Prime</a>.
%H A109024 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Emirp.html">Emirp</a>.
%H A109024 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Emirpimes.html">Emirpimes</a>.
%e A109024 a(1) = 126 is in this sequence because 126 = 2 * 3^2 * 7 is a 4-almost prime and reverse(126) = 621 = 3^3 * 23 is also a 4-almost prime.
%e A109024 a(2) = 225 is in this sequence because 225 = 3^2 * 5^2 is a 4-almost prime and reverse(225) = 522 = 2 * 3^2 * 29 is also a 4-almost prime. (That 225 and 522 are concatenated from entirely prime digits is a coincidence, as with 2223).
%t A109024 Select[Range[2226],PrimeOmega[#]==4 && PrimeOmega[FromDigits[Reverse[IntegerDigits[#]]]]==4 &&!PalindromeQ[#]&] (* _James C. McMahon_, Mar 07 2024 *)
%o A109024 (PARI) is(n) = {
%o A109024 	my(r = fromdigits(Vecrev(digits(n))));
%o A109024 	n!=r && bigomega(n) == 4 && bigomega(r) == 4
%o A109024 } \\ _David A. Corneth_, Mar 07 2024
%Y A109024 Cf. A006567, A097393, A109018, A109023, A109025, A109026, A109027, A109028, A109029, A109030, A109031.
%K A109024 nonn,base
%O A109024 1,1
%A A109024 _Jonathan Vos Post_, Jun 16 2005