OFFSET
0,2
FORMULA
G.f.: 6/(5-14*x+sqrt(1-8*x+4*x^2))=1/(1-2*x-x*F(x)), where F(x) is the g.f. of the sequence A007564.
a(n) ~ sqrt(1275+746*sqrt(3)) * (2*(2+sqrt(3)))^n / (121*sqrt(2*Pi)*n^(3/2)). - Vaclav Kotesovec, Apr 21 2015
a(n) = Sum_{k=0..n}((k+1)*(Sum_{j=0..k}(2^j*(-1)^(-k+j)*binomial(k-j,j)))*Sum_{j=0..n+1}(binomial(j,-n-k+2*j-2)*4^(-n-k+2*j-2)*3^(n-j+1)*binomial(n+1,j)))/(n+1). - Vladimir Kruchinin, Mar 08 2016
Conjecture: 2*n*a(n) +3*(-9*n+8)*a(n-1) +4*(28*n-39)*a(n-2) +4*(-43*n+81)*a(n-3) +64*(n-3)*a(n-4)=0. - R. J. Mathar, Sep 24 2016
EXAMPLE
a(2) = 10 because we have H1H1, H1H2, H2H1, H2H2, UDH1, UDH2, H1UD, H2UD, UDUD and UUDD.
MATHEMATICA
CoefficientList[Series[1/(1 - 2 x - x (1 + 2 x - Sqrt[1 - 8 x + 4 x^2])/(6 x)), {x, 0, 30}], x] (* Vincenzo Librandi, Apr 21 2015 *)
PROG
(Maxima)
a(n):=sum((k+1)*(sum(2^j*(-1)^(-k+j)*binomial(k-j, j), j, 0, k))*sum(binomial(j, -n-k+2*j-2)*4^(-n-k+2*j-2)*3^(n-j+1)*binomial(n+1, j), j, 0, n+1), k, 0, n)/(n+1); /* Vladimir Kruchinin, Mar 08 2016 */
CROSSREFS
KEYWORD
nonn
AUTHOR
José Luis Ramírez Ramírez, Apr 20 2015
EXTENSIONS
More terms from Vincenzo Librandi, Apr 21 2015
STATUS
approved