an+1=2^nan
an+1/an=2^n
a2/a1=2
a3/a2=2^2
.
an/an-1=2^(n-1)
两边相乘得an/a1=2^(1+2+3+...+n-1)
=2^(n*(n-1)/2)
an=2^(n*(n-1)/2)