Q:

Assume by/by way of /toward contradiction

What is the correct/best way to start a proof by contradiction? The following three ways are quite common:

Assume by contradiction that...
Assume, by way of contradiction, that...
Assume toward contradiction that...

Is there any preference? Specifically, are any of the grammatically or semantically incorrect?
Also, are there other, preferable ways?

A:

Mathematician here. The 2nd item in your list of suggestions is by far the best. The other two sound a little strange to me, and may very well be grammatically incorrect.
I will suggest a similar phrase: "Assume, for sake of contradiction, that...."  I use this in the courses I teach, and my students and I end up using the acronym AFSOC as a shorthand. 
I like this phrase because it accurately conveys what is going on in a proof by contradiction. That is, it really says, "I'm going to assume this is true right now, not because I really believe it, but because I will use it to find a contradiction."

