How would one say "Suppose that A?" I am looking for the way that this might be translated for mathematics.
There is no literal translation for "Suppose that A" as a complete sentence that sound natural to me. The best I can do is "Man nehme an, dass A" or "Nehmen wir an, dass A". The first sounds somewhat stilted, the second depends on you being fine using "we" in your text (literally it's "Let's suppose that A", which some people are fine using and some aren't.)
You can alse use "angenommen, dass A" or "unter der Annahme, dass A", which both translate to "supposing that A". (If A is a formula, you could only use "angenommen": "angenommen P = NP".) Another thing that might be good situationally is "Sei A X", which translates to "Let A be X". It sounds weird to my non-mathematician ears, but I believe people still use it.
You could add more context around your phrase, then it would be more clear what your query refers to exactly, but I suppose you refer to an assumption that is invalidated as part of the proof.
Then you should use:
Angenommen, X erfüllt Y; dann wäre .....
Suppose that X satisfies Y; then ..... would be .....