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 A000490 M5027 N2169 #46 Oct 26 2024 22:58:51
%S A000490 1,16,1280,249856,90767360,52975108096,45344872202240,
%T A000490 53515555843342336,83285910482761809920,165262072909347030040576,
%U A000490 407227428060372417275494400,1219998300294918683087199010816,4366953142363907901751614431559680,18406538229888710811704852978971181056
%N A000490 Generalized Euler numbers c(4,n).
%D A000490 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D A000490 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A000490 Matthew House, <a href="/A000490/b000490.txt">Table of n, a(n) for n = 0..194</a>
%H A000490 D. Shanks, <a href="http://dx.doi.org/10.1090/S0025-5718-1967-0223295-5">Generalized Euler and class numbers</a>. Math. Comp. 21 (1967) 689-694.
%H A000490 D. Shanks, <a href="http://dx.doi.org/10.1090/S0025-5718-1968-0227093-9">Corrigenda to: "Generalized Euler and class numbers"</a>, Math. Comp. 22 (1968), 699.
%H A000490 D. Shanks, <a href="/A000003/a000003.pdf">Generalized Euler and class numbers</a>, Math. Comp. 21 (1967), 689-694; 22 (1968), 699. [Annotated scanned copy]
%F A000490 a(n) = A000364(n)*16^n. - _Philippe Deléham_, Oct 27 2006
%F A000490 a(n) = (2*n)!*[x^(2*n)](sec(4*x)). - _Peter Luschny_, Nov 21 2021
%p A000490 egf := sec(4*x): ser := series(egf, x, 26):
%p A000490 seq((2*n)!*coeff(ser, x, 2*n), n = 0..11); # _Peter Luschny_, Nov 21 2021
%t A000490 a0 = 4; nmax = 20; km0 = nmax; Clear[cc]; L[a_, s_, km_] := Sum[ JacobiSymbol[-a, 2*k+1]/(2*k+1)^s, {k, 0, km}]; c[a_, n_, km_] := 2^(2*n +1)*Pi^(-(2*n)-1)*(2*n)!*a^(2*n+1/2)*L[a, 2*n+1, km] // Round; cc[km_] := cc[km] = Table[c[a0, n, km], {n, 0, nmax}]; cc[km0]; cc[km = 2 km0]; While[cc[km] != cc[km/2, km = 2 km]]; A000490 = cc[km] (* _Jean-François Alcover_, Feb 05 2016 *)
%t A000490 Range[0, 26, 2]! CoefficientList[Series[Sec[4 x], {x, 0, 26}], x^2] (* _Matthew House_, Oct 05 2024 *)
%Y A000490 Row 4 of A235605.
%Y A000490 Cf. A000187, A000436, A000318, A349264.
%K A000490 nonn,easy
%O A000490 0,2
%A A000490 _N. J. A. Sloane_
%E A000490 More terms from Kok Seng Chua (chuaks(AT)ihpc.nus.edu.sg), Jun 02 2000