OFFSET
0,4
COMMENTS
a(n) is the sum of the digits in ternary sequences of length n, in which each element of the alphabet, {0,1,2} appears at least once in the sequence.
Generally, the e.g.f. for such sum of n-ary sequences (taken on an alphabet of {0,1,2,...,n-1}) is binomial(n,2)*x*exp(x)*(exp(x)-1)^(n-1).
Cf. A058877 which is the sum of the digits in such binary sequences.
LINKS
Colin Barker, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (12,-58,144,-193,132,-36).
FORMULA
E.g.f.: 3*x*exp(x)*(exp(x)-1)^2.
a(n) = (3^n - 3*2^n + 3)*n. - Mark van Hoeij, May 13 2013
G.f.: 6*x^3*(11*x^2-12*x+3) / ((x-1)^2*(2*x-1)^2*(3*x-1)^2). - Colin Barker, Nov 30 2014
EXAMPLE
a(3)=18 because there are six length 3 sequences on {0,1,2} that contain at least one 0, at least one 1 and at least one 2: (0,1,2),(0,2,1),(1,0,2),(1,2,0),(2,0,1),(2,1,0). The digits sum to 18.
MATHEMATICA
Range[0, 20]! CoefficientList[Series[3x Exp[x](Exp[x]-1)^2, {x, 0, 20}], x]
Table[(3^n -3*2^n +3)*n, {n, 0, 30)] (* G. C. Greubel, Jan 24 2019 *)
PROG
(PARI) x='x+O('x^66); concat([0, 0, 0], Vec(serlaplace(3*x*exp(x)*(exp(x)-1)^2))) \\ Joerg Arndt, May 13 2013
(PARI) concat([0, 0, 0], Vec(6*x^3*(11*x^2-12*x+3)/((x-1)^2*(2*x-1)^2*(3*x-1)^2) + O(x^100))) \\ Colin Barker, Nov 30 2014
(PARI) vector(30, n, n--; (3^n - 3*2^n + 3)*n) \\ G. C. Greubel, Jan 24 2019
(Magma) [(3^n - 3*2^n + 3)*n: n in [0..30]]; // G. C. Greubel, Jan 24 2019
(Sage) [(3^n - 3*2^n + 3)*n for n in (0..30)] # G. C. Greubel, Jan 24 2019
(GAP) List([0..30], n -> (3^n - 3*2^n + 3)*n); # G. C. Greubel, Jan 24 2019
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Geoffrey Critzer, Dec 26 2010
STATUS
approved