If $\large a_1,a_2,a_3\ldots a{_n}$ is the series and $i = 1$ to $n,$ then the series is defined as $a_i=i{^2} + 1.$
i.e the $i^{th}$ term is $1$ plus the square of $i.$
Series will be as follows $:1^{2} +1 ,2^{2}+ 1,3^{2} +1,4^{2}+1\ldots n^{2} + 1$
$\qquad\qquad\qquad\qquad\quad\;\; 2,5,10,17,26,37,50,65$
Hence $64$ does not belong to the series.