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 A061380 #23 Sep 25 2024 15:48:19
%S A061380 0,1,3,6,10,66,105,120,153,190,210,231,300,351,406,465,630,703,741,
%T A061380 780,820,903,990,1035,1081,1326,1540,1770,1830,2016,2080,2556,2701,
%U A061380 2850,3003,3081,3160,3240,3403,3570,4005,4095,4560,4950,5050,5460,5671,6105
%N A061380 Triangular numbers with product of digits also a triangular number.
%H A061380 Michel Marcus, <a href="/A061380/b061380.txt">Table of n, a(n) for n = 1..10000</a>
%e A061380 153 is a triangular number and the product of digits 15 is also a triangular number.
%p A061380 q:= n-> (l-> issqr(1+8*mul(i,i=l)))(convert(n, base, 10)):
%p A061380 select(q, [seq(i*(i+1)/2, i=0..110)])[]; # _Alois P. Heinz_, Mar 17 2023
%t A061380 tn=Table[n (n+1)/2, {n, 0, 110}] ;Select[tn,MemberQ[tn,Times@@IntegerDigits[#]]&] (* _James C. McMahon_, Sep 25 2024 *)
%o A061380 (Magma) [t: n in [0..110] | IsSquare(8*p+1) where p is &*Intseq(t) where t is (n*(n+1) div 2)]; // _Bruno Berselli_, Jun 30 2011
%o A061380 (PARI) isok(k) = ispolygonal(k, 3) && ispolygonal(vecprod(digits(k)), 3); \\ _Michel Marcus_, Mar 17 2023
%Y A061380 Cf. A000217, A061867, A069077, A117710.
%K A061380 nonn,base,easy
%O A061380 1,3
%A A061380 _Amarnath Murthy_, May 02 2001
%E A061380 More terms from _Erich Friedman_, May 08 2001
%E A061380 Offset 1 from _Michel Marcus_, Mar 17 2023