OFFSET
0,4
COMMENTS
a(n) is the number of 3 X n matrices with the following properties:
i) Each row has at least one nonzero entry.
ii) Each column has exactly one nonzero entry.
iii) The nonzero entries in row m, 1 <= m <= 3, are in {1,2,...,m}.
This sequence counts such 3 X n matrices but the results are easily generalized for any such m X n matrix.
LINKS
Colin Barker, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (18,-121,372,-508,240).
FORMULA
E.g.f.: (exp(3*x)-1)*(exp(2*x)-1)*(exp(x)-1).
G.f.: 12*x^3*(3-18*x+20*x^2)/((1-x)*(1-2*x)*(1-4*x)*(1-5*x)*(1-6*x)). - Colin Barker, Nov 30 2014
For n > 0, a(n) = 1 + 2^n - 4^n - 5^n + 6^n. - Vaclav Kotesovec, Dec 01 2014
a(n) = 18*a(n-1) - 121*a(n-2) + 372*a(n-3) - 508*a(n-4) + 240*a(n-5). - Vaclav Kotesovec, Dec 01 2014
MATHEMATICA
a=Exp[x]-1; b=Exp[2x]-1; c=Exp[3x]-1; Range[0, 20]! CoefficientList[Series[a b c, {x, 0, 20}], x]
PROG
(PARI) concat([0, 0, 0], Vec(-12*x^3*(20*x^2-18*x+3)/((x-1)*(2*x-1)*(4*x-1)*(5*x-1)*(6*x-1)) + O(x^30))) \\ Colin Barker, Dec 01 2014
(Magma) m:=30; R<x>:=PowerSeriesRing(Rationals(), m); b:=Coefficients(R!( (Exp(3*x)-1)*(Exp(2*x)-1)*(Exp(x)-1) )); [0, 0, 0] cat [Factorial(n+2)*b[n]: n in [1..m-3]]; // G. C. Greubel, Jan 26 2019
(Sage) m = 30; T = taylor((exp(3*x)-1)*(exp(2*x)-1)*(exp(x)-1), x, 0, m); [factorial(n)*T.coefficient(x, n) for n in (0..m)] # G. C. Greubel, Jan 26 2019
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Geoffrey Critzer, Dec 25 2010
STATUS
approved