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 A110106 a(n) is the number of coverings of 1..n by cyclic words of length 3n, such that each value from 1 to n appears precisely twice. That is, the union of all the letters in all of the words of a given covering is the multiset {1,1,2,2,...,n,n}. Repeats of words are allowed in a given covering. 3
 1, 6, 3960, 24151680, 577882166400, 38039350155206400, 5605398331566095462400, 1614162682147590619140096000, 824800497779996439355497811968000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS P-recursive. LINKS FORMULA Differential equation satisfied by F(t)=sum a(n) t^(3n)/(3n!) {F(0) = 1, (6*t^2-12*t^5+t^8)*F(t) + (-4*t^6-2+16*t^3)*(d/dt)F(t) + 4*t^4*(d^2/dt^2)F(t)}; recurrence satisfied by a(n): {(40320 + 328752*n + 78732*n^7 + 6561*n^8 + 1816668*n^3 + 1818369*n^4 + 1102248*n^5 + 398034*n^6 + 1063116*n^2)*a(n) + (-161280 - 508608*n - 453600*n^3 - 173340*n^4 - 34992*n^5 - 2916*n^6 - 661104*n^2)*a(n+1) + (12432 + 20070*n + 12114*n^2 + 3240*n^3 + 324*n^4)*a(n+2) - 2*a(n+3), a(1) = 6, a(0) = 1, a(2) = 3960}. EXAMPLE a(1)=6: {123, 132} {112, 233} {113, 322} {133, 122} {123, 123} {132, 132}. CROSSREFS Cf. A052205, A110104, A110105, A108242. Sequence in context: A048542 A140173 A158663 * A024087 A161845 A317485 Adjacent sequences:  A110103 A110104 A110105 * A110107 A110108 A110109 KEYWORD easy,nonn AUTHOR Marni Mishna, Jul 11 2005 STATUS approved

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Last modified June 24 00:06 EDT 2021. Contains 345403 sequences. (Running on oeis4.)