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 A181615 #21 Jun 29 2024 16:03:07
%S A181615 1,2,2,0,4,4,0,2,8,8,0,4,7,6,2,0,2,5,2,4,2,0,4,1,4,8,4,1,0,2,2,8,2,8,
%T A181615 2,18,0,4,4,6,4,1,4,14,6,0,2,8,2,8,2,8,6,12,18,0,4,7,4,2,4,16,12,24,
%U A181615 10,1,0,2,5,8,4,8,11,2,23,20,2,8,0,4,1,6,8,1,1,4,21,14,4
%N A181615 Triangle read by rows: T(n,k) = 2^(n-1) mod semiprime(k).
%e A181615 Triangle begins:
%e A181615 1;
%e A181615 2, 2;
%e A181615 0, 4, 4;
%e A181615 0, 2, 8, 8;
%e A181615 0, 4, 7, 6, 2;
%e A181615 ...
%t A181615 Table[PowerMod[2, n-1, #[[;;n]]], {n, Length[#]}] & [Select[Range[50], PrimeOmega[#] == 2 &]] (* _Paolo Xausa_, Jun 29 2024 *)
%o A181615 (PARI) trg(nn) = {semip = select(n->bigomega(n) == 2, vector(nn, i, i)); for (n = 1, #semip, for (k = 1, n, print1(2^(n-1) % semip[k], ", ");); print(););} \\ _Michel Marcus_, Sep 11 2013
%Y A181615 Cf. A173622, A181670.
%K A181615 nonn,tabl
%O A181615 1,2
%A A181615 _Juri-Stepan Gerasimov_, Dec 02 2010
%E A181615 Corrected by _T. D. Noe_, Dec 02 2010