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 A047893 #20 Mar 20 2025 03:21:38 %S A047893 1,1,2,4,5,7,9,11,13,15,17,20,22,25,27,30,32,35,38,41,44,46,49,52,55, %T A047893 58,61,64,68,71,74,77,80,84,87,90,94,97,100,104,107,111,114,118,121, %U A047893 125,128,132,135,139,143,146,150,154,157,161,165,168,172,176,180,183 %N A047893 Number of decimal digits of Euler (Zig) numbers A000364. %D A047893 J. Peters and J. Stein, Mathematische Tafeln, Revised Russian Edition, Moscow, 1968. %H A047893 Amiram Eldar, <a href="/A047893/b047893.txt">Table of n, a(n) for n = 1..10000</a> %H A047893 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/EulerNumber.html">Euler Number</a>. %F A047893 a(n) = A055642(A000364(n)) = floor(log_10(|E(2n)|)) + 1 for n >=2, where E is Euler's E function. %e A047893 a(4) = floor(log_10(1385)) + 1 = 4, E(4) = 1385, the 4th Euler number. %t A047893 a[n_] := IntegerLength[EulerE[2*n]]; Array[a, 100] (* _Amiram Eldar_, Mar 20 2025 *) %o A047893 (PARI) a(n) = #Str(subst(bernpol(2*n+1), 'x, 1/4)*4^(2*n+1)*(-1)^(n+1)/(2*n+1)); %Y A047893 Cf. A000364, A034971, A034972, A000182 (tangent numbers), A047894. %K A047893 nonn,base %O A047893 1,3 %A A047893 _Labos Elemer_ %E A047893 a(1) corrected by _Amiram Eldar_, Mar 20 2025