cp's OEIS Frontend

A131421 Triangle read by rows (n>=1, 1<=k<=n): T(n,k) = 2*(n+k) - 3.

%I A131421 #28 Oct 06 2017 14:59:08
%S A131421 1,3,5,5,7,9,7,9,11,13,9,11,13,15,17,11,13,15,17,19,21,13,15,17,19,21,
%T A131421 23,25,15,17,19,21,23,25,27,29,17,19,21,23,25,27,29,31,33,19,21,23,25,
%U A131421 27,29,31,33,35,37,21,23,25,27,29,31,33,35,37,39,41
%N A131421 Triangle read by rows (n>=1, 1<=k<=n): T(n,k) = 2*(n+k) - 3.
%C A131421 Row sums = A000567, the octagonal numbers: (1, 8, 21, 40, 65, 96,...).
%H A131421 Michael De Vlieger, <a href="/A131421/b131421.txt">Table of n, a(n) for n = 1..11325</a> (rows 1 <= n <= 150).
%F A131421 (A000012 * A127775) + (A127775 * A000012) - A000012, where all the sequences and the result are interpreted as infinite lower triangular matrices.
%e A131421 First few rows of the triangle are:
%e A131421 1;
%e A131421 3, 5;
%e A131421 5, 7, 9;
%e A131421 7, 9, 11, 13;
%e A131421 9, 11, 13, 15, 17;
%e A131421 11, 13, 15, 17, 19, 21;
%e A131421 13, 15, 17, 19, 21, 23, 25;
%e A131421 ...
%t A131421 Table[2 (n + k) - 3, {n, 150}, {k, n}] // Flatten (* _Michael De Vlieger_, Oct 06 2017 *)
%o A131421 (PARI) tabl(nn) = {ma = matrix(nn, nn, n, k, (k<=n)); mb = matrix(nn, nn, n, k, (2*n - 1)*(k==n)); mr = ma*mb + mb*ma - ma; for (n = 1, nn, for (k = 1, n, print1(mr[n, k], ", ");); print(););} \\ _Michel Marcus_, Mar 04 2014
%Y A131421 Cf. A127775, A000567.
%K A131421 nonn,tabl,easy
%O A131421 1,2
%A A131421 _Gary W. Adamson_, Jul 10 2007
%E A131421 Corrected and extended by _Michel Marcus_, Mar 04 2014
%E A131421 New name from _Andrey Zabolotskiy_, Oct 06 2017