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 A055973 Number of unlabeled digraphs with loops (relations) on n nodes and with an even number of arcs. 2
 1, 1, 6, 52, 1540, 145984, 48467296, 56141454464, 229148555640864, 3333310786076963968, 174695272746896566439424, 33301710992539090379269318144, 23278728241293494584257987458067456, 60084295633556503802059558812644803074048, 576025077880237078776946976495257043823396069376 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Also relations considered equivalent when isomorphic and when complemented. - Christian G. Bower, Jan 05 2004 LINKS Andrew Howroyd, Table of n, a(n) for n = 0..50 FORMULA a(2*n) = (A000595(2*n) + A047832(n))/2; a(2*n+1) = A000595(2*n+1)/2. a(n) = (A000595(n) + A000171(2*n+1))/2. 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 CROSSREFS Cf. A000171, A000595, A007869, A047832, A054928, A055974. Sequence in context: A166889 A164894 A027835 * A223345 A362907 A369506 Adjacent sequences: A055970 A055971 A055972 * A055974 A055975 A055976 KEYWORD easy,nonn AUTHOR Vladeta Jovovic, Jul 19 2000 EXTENSIONS a(0)=1 prepended and terms a(13) and beyond from Andrew Howroyd, Apr 19 2020 STATUS approved

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Last modified August 15 03:10 EDT 2024. Contains 375172 sequences. (Running on oeis4.)