SOCP portfolio optimisation notebook - issue with conversion to pdf

[mikecroucher]

The pdf produced by running

jupyter nbconvert --to pdf portfolio_optimisation_using_socp.ipynb

on

https://github.com/numericalalgorithmsgroup/NAGPythonExamples/blob/master/local_optimisation/SOCP/portfolio_optimisation_using_socp.ipynb

has a problem with citations. We get [?] instead of [1], [2] etc