A181305 - OEIS

%I #7 Jul 28 2015 16:52:52

%S 0,1,5,24,104,432,1736,6820,26332,100308,377996,1411844,5234428,

%T 19285252,70670972,257766212,936336572,3388962884,12226547132,

%U 43983439684,157814634684,564917186372,2017873643708,7193745818436

%N Number of increasing columns in all 2-compositions of n. A 2-composition of n is a nonnegative matrix with two rows, such that each column has at least one nonzero entry and whose entries sum up to n.

%C Also, number of odd entries in the top rows of all 2-compositions of n. A 2-composition of n is a nonnegative matrix with two rows, such that each column has at least one nonzero entry and whose entries sum up to n.

%C a(n)=Sum(A181304(n,k), k=0..n).

%C For the case of the even entries see A181337.

%D G. Castiglione, A. Frosini, E. Munarini, A. Restivo and S. Rinaldi, Combinatorial aspects of L-convex polyominoes, European Journal of Combinatorics, 28, 2007, 1724-1741.

%H <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (7, -12, -4, 12, -4).

%F G.f. = z(1-z)^2/[(1+z)(1-4z+2z^2)^2].

%e a(1)=1 because in the 2-compositions of 1, namely (0/1) and (1/0) we have only one increasing column (the 2-compositions are written as (top row / bottom row).

%e a(2)=5 because in (0/2),(1/1),(2,0),(1,0/0,1),(0,1/1,0),(1,1/0,0), and (0,0/1,1) (the 2-compositions are written as (top row/bottom row)) we have 0+1+0+1+1+2+0=5 odd entries.

%p g := z*(1-z)^2/((1+z)*(1-4*z+2*z^2)^2): gser := series(g, z = 0, 30): seq(coeff(gser, z, k), k = 0 .. 27);

%Y Cf. A181304, A181337.

%K nonn

%O 0,3

%A _Emeric Deutsch_, Oct 13 2010

%E Edited by _N. J. A. Sloane_, Oct 15 2010