In a non-zero common difference arithmetic sequence $a_{1} \cdot a_{2}, \cdots, a_{1990}$, $a_{i} a,>0,(i, j=1,2, \cdots, 1990)$. Let $b_{1}=a_{1} \cdot a_{1990}, b_{2}=a_{2} \cdot a_{1989}, \cdots, b_{k}=a_{k} \cdot a_{1990-k+1}(k=1,2, \cdots, 1990)$, then the largest term in $\left\{b_{k}\right\}$ is $\qquad$ .
$53 b_{995}$ and $b_{996}$. According to the properties of an arithmetic sequence, we have $a_{1}+a_{1990}=a_{2}+a_{1989}=\cdots=a_{k}+$ $a_{1990-k+1}$, the sum of two numbers with the same sign is a constant, the smaller the difference, the greater the product. The smallest difference is between $a_{995}$ and $a_{996}$, their product $b_{995}=a_{995} \cdot a_{996}=a_{996} \cdot a_{995}=b_{996}$ are the two larger values.