OFFSET
1,3
COMMENTS
Same as number of strings over GF(3) of length n with trace 0 and subtrace 0.
LINKS
Max Alekseyev, PARI/GP scripts for miscellaneous math problems
Index entries for linear recurrences with constant coefficients, signature (6,-15,27,-36,27).
FORMULA
a(n; t, s) = a(n-1; t, s) + a(n-1; t+2, s+2t+1) + a(n-1; t+1, s+t+1) where t is the trace and s is the subtrace.
G.f.: q*(21*q^4-21*q^3+12*q^2-5*q+1)/[(1-3q)(1+3q^2)(1-3q+3q^2)]. - Lawrence Sze, Oct 24 2004
EXAMPLE
a(3;0,0)=3 since the three ternary strings of trace 0, subtrace 0 and length 3 are { 000, 111, 222 }.
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Frank Ruskey and Nate Kube, Aug 15 2002
EXTENSIONS
Terms a(21) onward from Max Alekseyev, Apr 09 2013
STATUS
approved