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 A056372 #19 Dec 01 2017 20:14:58
%S A056372 3,9,18,54,72,405,390,1944,3411,14985,17802,139968,133104,798525,
%T A056372 1804518,5454378,8072532,64599849,64573626,437732424,872157294,
%U A056372 3138159429,4279259574,35362084140,42364514403,211822562025,423646166250
%N A056372 Number of step shifted (decimated) sequences using a maximum of three different symbols.
%C A056372 See A056371 for an explanation of step shifts.
%H A056372 G. C. Greubel, <a href="/A056372/b056372.txt">Table of n, a(n) for n = 1..2000</a>
%H A056372 R. C. Titsworth, <a href="http://projecteuclid.org/euclid.ijm/1256059671">Equivalence classes of periodic sequences</a>, Illinois J. Math., 8 (1964), 266-270.
%F A056372 The cycle index is implicit in Titsworth.
%F A056372 Sequences A056372-A056375 fit a general formula, implemented in PARI/GP as follows: { a(m,n) = sum(k=1, n, if(gcd(k, n)==1, m^sumdiv(n, d, eulerphi(d)/znorder(Mod(k, d))), 0); ) / eulerphi(n) }. - _Max Alekseyev_, Nov 08 2007
%t A056372 a[m_, n_] := (1/EulerPhi[n])*Sum[If[GCD[k, n] == 1, m^DivisorSum[n, EulerPhi[#] / MultiplicativeOrder[k, #]&], 0], {k, 1, n}]; Table[a[3, n], {n, 1, 27}] (* _Jean-François Alcover_, Dec 04 2015 *)
%Y A056372 Cf. A056411.
%Y A056372 A row or column of A132191.
%K A056372 nonn
%O A056372 1,1
%A A056372 _Marks R. Nester_
%E A056372 More terms from _Max Alekseyev_, Nov 08 2007