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 A056391 Number of step shifted (decimated) sequence structures using a maximum of two different symbols. 287
 1, 2, 3, 6, 6, 20, 14, 48, 52, 140, 108, 624, 352, 1400, 2172, 4464, 4116, 22112, 14602, 68016, 88376, 209936, 190746, 1075200, 839128, 2797000, 3730584, 11276704, 9587580, 67195520, 35792568 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS See A056371 for an explanation of step shifts. Permuting the symbols will not change the structure. Also, number of circulant digraphs on n vertices up to Cayley isomorphism. Two circulant graphs are Cayley isomorphic if there is a d, which is necessarily prime to n, that transforms through multiplication modulo n the step values of one graph into those of the other. For squarefree n this is the only way that two circulant graphs can be isomorphic (see A049297). - Andrew Howroyd, Apr 20 2017 REFERENCES M. R. Nester (1999). Mathematical investigations of some plant interaction designs. PhD Thesis. University of Queensland, Brisbane, Australia. LINKS G. C. Greubel, Table of n, a(n) for n = 1..1000 (terms 1..200 from Andrew Howroyd) Andrew Howroyd, Polya Enumeration in PARI (many sequences included) Marks R. Nester, Mathematical investigations of some plant interaction designs, Chapter 2, Finite and Periodic Sequences, plus Notes and Errata. FORMULA Use de Bruijn's generalization of Polya's enumeration theorem as discussed in reference. a(n) = A056371(n) / 2. - Andrew Howroyd, Apr 20 2017 a(n) = A288620(n, 2) + 1. - Andrew Howroyd, Jun 13 2017 MATHEMATICA a[m_, n_] := (1/EulerPhi[n])*Sum[If[GCD[k, n] == 1, m^DivisorSum[n, EulerPhi[#]/MultiplicativeOrder[k, #]&], 0], {k, 1, n}]; a[n_] := a[2, n]/2; Array[a, 40] (* Jean-François Alcover, Jun 12 2017 *) PROG (PARI) a(n)=sum(k=1, n, if(gcd(k, n)==1, 2^(sumdiv(n, d, eulerphi(d)/znorder(Mod(k, d)))-1), 0))/eulerphi(n); \\ Andrew Howroyd, Apr 20 2017 (PARI) \\ alternative using Polya enumeration functions (see attachment) a(n) = NonequivalentStructs(StepShiftPerms(n), 2); \\ Andrew Howroyd, Oct 01 2017 CROSSREFS Row 2 of A285522. Cf. A056371, A049297, A285620, A049287, A288620. Sequence in context: A223547 A049297 A285664 * A056430 A089878 A057545 Adjacent sequences:  A056388 A056389 A056390 * A056392 A056393 A056394 KEYWORD nonn AUTHOR STATUS approved

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Last modified October 19 16:17 EDT 2019. Contains 328223 sequences. (Running on oeis4.)