This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A381920 #9 Mar 16 2025 22:14:39
%S A381920 1326,1770,2145,2415,3003,3486,4186,5565,6670,7626,8385,8646,9730,
%T A381920 13695,17205,17578,24531,25878,27730,28203,35245,35778,37401,42486,
%U A381920 47278,47895,51681,59685,60378,63190,63903,66795,72010,74305,75855,81406,84666,87153,91378,95703
%N A381920 Hexagonal numbers that are products of exactly four distinct primes.
%e A381920 1326 = 2*3*13*17 is the product of 4 distinct primes and the 26th hexagonal number hex(26) = 26(2*26-1).
%e A381920 1770 = 2*3*5*59 is the product of 4 distinct primes and the 30th hexagonal number hex(30) = 30(2*30-1).
%e A381920 2145 = 3*5*11*13 is the product of 4 distinct primes and the 33rd hexagonal number hex(33) = 33(2*33-1).
%t A381920 Select[Table[n*(2*n-1), {n, 1, 220}], FactorInteger[#][[;; , 2]] == {1, 1, 1, 1} &] (* _Amiram Eldar_, Mar 10 2025 *)
%Y A381920 Intersection of A000384 and A046386.
%Y A381920 Cf. A129521, A380007.
%K A381920 nonn
%O A381920 1,1
%A A381920 _Massimo Kofler_, Mar 10 2025