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A224289
Number of permutations of length n containing exactly 1 occurrence of 123 and 2 occurrences of 132.
0
0, 0, 0, 2, 8, 26, 79, 232, 664, 1856, 5072, 13568, 35584, 91648, 232192, 579584, 1427456, 3473408, 8359936, 19922944, 47054848, 110231552, 256311296, 591921152, 1358430208, 3099590656, 7034896384, 15888023552, 35718692864, 79960211456, 178291474432, 396076515328, 876844417024
OFFSET
1,4
FORMULA
a(n) = 2^(-8+n)*(-136+70*n-11*n^2+n^3) for n>3. G.f.: -x^4*(x^3-10*x^2+8*x-2) / (2*x-1)^4. - Colin Barker, Apr 14 2013
MAPLE
# Programs can be obtained from author's personal website.
MATHEMATICA
LinearRecurrence[{8, -24, 32, -16}, {0, 0, 0, 2, 8, 26, 79}, 40] (* Harvey P. Dale, Jun 23 2017 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Brian Nakamura, Apr 03 2013
STATUS
approved