数列1 2 12 216 11664 .的通项公式是什么?

a1= 1= 2^0 * 3^0 ( 0)
a2= 2= 2^1 * 3^0 (0+0= 0)
a3= 12= 2^2 * 3^1 (1+0= 1=1)
a4= 216= 2^3 * 3^3 (2+1= 3=1+2)
a5=11664=2^4 * 3^6 (3+3= 6=1+2+3)
a6= 2^5 * 3^10 (4+6= 10=1+2+3+4)
a7= 2^6 * 3^15 (5+10=15=1+2+3+4+5)
.
an=2^(n-1) * 3^[(n-1)(n-2)/2]