11. Let $f(x)=2^{x}+x^{2}$, write a sequence such that the sum of its first $n$ terms has the form of $f(n)$, $a_{n}=$ $\qquad$
11. $a_{n}=\left\{\begin{array}{l}3, n=1 ; \\ 2^{n-1}+2 n-1, n \geqslant 2 .\end{array}\right.$
$f(n)=2^{n}+n^{2},\{a_{n}\}$ satisfies $a_{1}=f(1)=3, a_{n}=f(n)-f(n-1)=2^{n-1}+2 n-1(n \geqslant 2)$.