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A055973
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Number of unlabeled digraphs with loops (relations) on n nodes and with an even number of arcs.
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2
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1, 1, 6, 52, 1540, 145984, 48467296, 56141454464, 229148555640864, 3333310786076963968, 174695272746896566439424, 33301710992539090379269318144, 23278728241293494584257987458067456, 60084295633556503802059558812644803074048, 576025077880237078776946976495257043823396069376
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OFFSET
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0,3
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COMMENTS
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Also relations considered equivalent when isomorphic and when complemented. - Christian G. Bower, Jan 05 2004
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LINKS
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FORMULA
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a(n) = sum {1*s_1+2*s_2+...=n, 1*t_1+2*t_2=2} (fixA[s_1, s_2, ...;t_1, t_2]/(1^s_1*s_1!*2^s_2*s_2!*...*1^t_1*t_1!*2^t_2*t_2!)) where fixA[...] = prod {i, j>=1} ( (sum {d|lcm(i, j)} (d*t_d))^(gcd(i, j)*s_i*s_j)) - Christian G. Bower, Jan 05 2004
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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EXTENSIONS
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a(0)=1 prepended and terms a(13) and beyond from Andrew Howroyd, Apr 19 2020
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STATUS
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approved
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