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A085584
Number of (3412,2341)-, (3412,4123)- and (3412,52341)-avoiding involutions in S_n.
0
1, 2, 4, 9, 20, 45, 100, 222, 491, 1086, 2401, 5310, 11744, 25977, 57460, 127101, 281144, 621882, 1375579, 3042726, 6730385, 14887338, 32930188, 72840249, 161119700, 356390301, 788321020, 1743734406, 3857070395, 8531684622
OFFSET
1,2
FORMULA
G.f. (1-x^2)*(1-x)^2/(1-3*x+x^2+3*x^3-3*x^4).
MATHEMATICA
CoefficientList[Series[(1-x^2)(1-x)^2/(1-3x+x^2+3x^3-3x^4), {x, 0, 40}], x] (* Harvey P. Dale, May 28 2017 *)
CROSSREFS
Sequence in context: A144686 A108469 A144685 * A369614 A080019 A052534
KEYWORD
nonn,easy
AUTHOR
Ralf Stephan, Jul 06 2003
STATUS
approved