%I
%S 3,24,528,31968,5332224,2450774016,3104112826368
%N Consider triangles stacked so the kth row has 2*k1 triangles. a(n) is the number of ways to color each triangle in the first n rows using three colors with the restriction that adjacent triangles must be different colors. (Triangles are adjacent if they share a side.)
%H Project Euler, <a href="https://projecteuler.net/problem=189">Problem 189: Tricolouring a triangular grid</a>
%e Illustration of first two rows:
%e /\
%e / \
%e / 1 \
%e / \
%e /________\
%e /\ /\
%e / \ / \
%e / 2 \ 3 / 4 \
%e / \ / \
%e /________\/________\
%e Triangles sharing a side are considered adjacent; e.g., 1 & 3 are adjacent in the figure, 1 & 2 are not.
%K nonn
%O 1,1
%A Chris Cole (chris(AT)questrel.com), Oct 20 2009
%E Edited by _Jon E. Schoenfield_, Jul 08 2017
%E Name simplified by _Charles R Greathouse IV_, Jul 09 2017
