Corpus of mathematical papers and proofs
Mathematical proofs can be found everywhere, even in the pudding. A more healthy place to look for proofs would be in research papers. The e-print server arXiv.org has a large collection of such papers and even share them publicly. So far, I have found over a million proofs in these papers. I am not sure whether I would have gathered the same amount of proofs, by eating 100 puddings a day over my entire life.
The lingua franca of science: ‘English’
Another useful thing to know is that the texts that are accessible on the arXiv.org are almost exclusively (I counted 99.4%) written in English. This might come in handy if you want to do some quantitative linguistics on that corpus.
Who uses proof by contradiction
To use a method known as proof by contradiction, is a very common phenomenon among scientists. Well, ‘pure’ mathematicians tend to use them more then ‘applied’, but still.