A strobogrammatic number which reads the same upside down, e.g. 619. On the other hand, a triangular number is a number of the form n(n+1) / 2 for some n \in \mathbb{N}, e.g. 15 (therefore, the i^{\text {th }} triangular number T_{i} is the sum of 1 through i ). Let a be the third strobogrammatic prime number. Let b be the smaller number of the smallest pair of triangular numbers whose sum and difference are also triangular numbers. What is the value of a b?