An analyst, de Branges has made incursions into real, functional, complex, harmonic (Fourier) and Diophantine analyses.

It took verification by a team of mathematicians at Steklov Institute of Mathematics in Leningrad to validate de Branges' proof, a process that took several months and led later to significant simplification of the main argument.

That original preprint suffered a number of revisions until it was replaced in December 2007 by a much more ambitious claim, which he had been developing for one year in the form of a parallel manuscript.

De Branges, incidentally, also claims that his new proof represents a simplification of the arguments present in the removed paper on the classical Riemann hypothesis, and insists that number theorists will have no trouble checking it.

Li and Conrey do not assert that de Branges' mathematics are wrong, only that the conclusions he drew from them in his original papers are, and that his tools are therefore inadequate to address the problems in question.

[5] Meanwhile, the "apology" has become a diary of sorts, in which he also discusses the historical context of the Riemann hypothesis, and how his personal story is intertwined with the proofs.

The particular analysis tools he has developed, although largely successful in tackling the Bieberbach conjecture, have been mastered by only a handful of other mathematicians (many of whom have studied under de Branges).

This poses another difficulty to verification of his current work, which is largely self-contained: most research papers de Branges chose to cite in his claimed proof of the Riemann hypothesis were written by himself over a period of forty years.

A simple search in the arXiv yields several claims of proofs, some of them by mathematicians working at academic institutions, that remain unverified and are usually dismissed by mainstream scholars.

This shows that de Branges' apparent estrangement is not an isolated case, but he is probably the most renowned professional to have a current unverified claim.