y=2^m+1,
2^m=y-1
m=log2[y-1]
把m带入x的表达式得
x=3+16^m
=3+16^{l0g2[y-1]}
=3+2^{4log2[y-1]}
=3+2^{log2[y-1]^4}
=3+(y-1)^4
所以 x=(y-1)^4+3