What are some examples of mathematicians who had an unconventional education and yet, went on to make an impact on mathematics? Here is an example: [Edward Witten][1]. He does not have a formal undergraduate degree in mathematics or physics but won the Fields medal.

More precisely, I am looking for examples of mathematicians whose undergraduate mathematical education was in a different field or was hindered by circumstances like war and poverty or did not have a formal degree as was the case of [Srinivasa Ramanujan][2].


  [1]: http://en.wikipedia.org/wiki/Edward_Witten
  [2]: http://en%20wikipedia%20org%20wiki%20srinivasa_ramanujan/