In reply to Rick Wicklin.

Thanks, for your help.

I posted on math.stackexchange.com about this.
Here's the link : https://math.stackexchange.com/questions/2626889/finding-the-eigenvalues-between-smallest-and-largest-eigenvalues-of-a-sparse-mat

I guess, I need to study more and try some other method to get this done.

Thanks again.