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In addition to what Marius said, I would mention:
- Riemannian geometry preceded its applications to general relativity
- Hilbert spaces preceded their applications to quantum theory
- Clifford algebras and spinors preceded their applications to relativistic quantum theory
- connections on fiber bundles preceded their applications to Yang-Mills theory
- holonomy groups preceded their applications to gauge theory, to Wilson loops and to loop quantum gravity
- representations of Lie group preceded their applications to particle physics
- topos theory preceded its applications to quantum theory obtained by Chris Isham
- the particular Kaehler manifolds named Calabi-Yau manifolds preceded their applications to string theory
I agree that mathematics originated from practical necessities, which come from the physical world. But mathematicians are playful species, and they like to explore platonic worlds as well. For some reason, their explorations anticipated many of the necessities of physics. Or maybe physicists find easier to borrow from mathematics, rather than making their own tools ;-). Or when they do, the tools are often full of divergences and singularities, and are inconsistent. As John Baez said once, it is the job of mathematicians to eliminate these inconsistencies. So I would say that both physicists and mathematicians have their equally important role.
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Lebniz's monadology finds also applications in Haskell (programming language). It also influenced Whitehead, and through him some applications to quantum theory.
Best regards,
Cristi