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assigned tag 0EW1 2018-08-29 2bcd4d3
Tags: Added new tags
created statement with label lemma-blow-up-square-ph in flat.tex 2018-08-29 9946ff8
Blow up squares and ph topology

Skeleton section.

It is NOT possible to characterize ph sheaves in terms of fppf coverings
and blow up squares. The problem is that there are proper surjective
morphisms which cannot be made flat by a finite stratification of the
affine base... Example

Base is the spectrum of A = Z[x_0, x_1, x_2, x_3, ...]
Source is the Proj of the graded ring A[S, T]/I where I is generated by
the elements

x_0(S - T), x_1(S^2 - T^2), x_2(S^4 - T^4), x_3(S^8 - T^8), ...

Over the stratum where x_0 = ... = x_{e - 1} = 0 but where x_e is
nonzero we get a degree 2^e cover...