What do we know about schizophrenia and mathematics? Schizophrenia is usually associated with intellectual problems. I don't think that cognitive deficits are only associated with vulnerability to schizophrenia, though there is the much cited


  Russell et al (1997) recently proposed that intellectual decline in
  schizophrenia was a nothing more than a ‘myth’. They followed up
  patients who had received an intelligence test as a child, who then
  developed schizophrenia, and had their intelligence re-tested some 19
  years later. There were no differences between the child and adult
  IQs. However, others have criticised this study on the grounds that
  the sample was unrepresentative in that the individuals had presented
  to child psychiatry units and were far more intellectually impaired
  than subjects in other studies of children who subsequently developed
  schizophrenia.


From my research prior to this question, it seems that there is a loss of ability, but likely not chronic decline over the course of the illness, one before or during the transition to psychosis. Another well known meta review claims this, that there's


  no support for hypothesizing a degenerative
  process in schizophrenia once positive symptoms appear... a static encephalopathy rather than a degenerative process


I've seen it said that it is centered on mathematical ability.

What do we know about mathematical ability pre and post psychosis, in schizophrenia? e.g., was John Nash a worse mathematician after he went mad?

References

Russell, A. J., et al. (1997) Schizophrenia and the myth of intellectual decline. American Journal of Psychiatry, 154(5), pp 635–639.
DOI: 10.1176/ajp.154.5.635 PMID: 9137118 WOS: A1997WX13000009
I spent times of the order of five to eight months in hospitals in New
  Jersey, always on an involuntary basis and always attempting a legal
  argument for release. And it did happen that when I had been long
  enough hospitalized that I would finally renounce my delusional
  hypotheses and revert to thinking of myself as a human of more
  conventional circumstances and return to mathematical research. In
  these interludes of, as it were, enforced rationality, I did succeed
  in doing some respectable mathematical research.


Seems like he himself is saying his research suffered while unwell and not staying in institutions. 

However, the standard deficit in IQ, of it seems around 10 points, may be inconsequential in someone like Nash. Even if the executive function and memory impairments are more significant.