%I A056120
%S A056120 1,1,7,27,108,432,1728,6912,27648,110592,442368,1769472,7077888,
%T A056120 28311552,113246208,452984832,1811939328,7247757312,28991029248,
%U A056120 115964116992,463856467968,185542587182
%N A056120 a(n)=(3^3)*4^(n-3); a(0)=1, a(1)=1.
%C A056120 For n>=3, a(n) is equal to the number of functions f:{1,2,...,n}->{1,
               2,3,4} such that for fixed, different x_1, x_2, x_3 in {1,2,...,n} 
               and fixed y_1, y_2, y_3 in {1,2,3,4} we have f(x_i)<>y_i, (i=1,2,
               ...,n). - Milan R. Janjic (agnus(AT)blic.net), May 13 2007
%H A056120 Milan Janjic, <a href="http://www.pmfbl.org/janjic/">Enumerative Formulas 
               for Some Functions on Finite Sets</a>
%F A056120 a(n)=4a(n-1)+[(-1)^n]*C(3, 3-n). G.f.(x)=(1-x)^3/(1-4x).
%Y A056120 Cf. A055841.
%Y A056120 First differences of A002063.
%Y A056120 Sequence in context: A054485 A090856 A055917 this_sequence A048711 A118101 
               A147996
%Y A056120 Adjacent sequences: A056117 A056118 A056119 this_sequence A056121 A056122 
               A056123
%K A056120 easy,nonn
%O A056120 0,3
%A A056120 Barry E. Williams, Jul 05 2000