I've read Wikipedia about it and it says:
In mathematics before the 1970s, the term umbral calculus referred to the surprising similarity between seemingly unrelated polynomial equations and certain shadowy techniques used to 'prove' them.
- What are these techniques?
These similarities allow one to construct umbral proofs, which, on the surface cannot be correct, but seem to work anyway.
- What does "seem to work" mean here?
- It seems that umbral calculus is a mathematical idea with almost no uses, why? (At least it's not so famous as calculus and algebra, for example.)