A calculator is an 'indispensable' tool 'for' solving these problems.
She is an 'indispensable' [='essential'] part of the team.
She is 'indispensable to' the team.
(..) I knew the chief would have to reconsider. He did not get where he is by ignoring a man's qualifications. He knows that I am unique, and therefore indispensable. We are much the same, the chief and I. But of course, Cato, all great detectives have one thing in common. And you know (..)1975 The Return of the Pink Panther