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Mathematician, researching in Algebraic Geometry and related topics.
2h

comment 
Find all solutions in positive integers of the diophantine equation $w^2+x^2+y^2=z^2$
Hint: sthereographic projection. So said, I'm voting to close since this is not a researchlevel question. 
2d

comment 
singular point of a complete intersection surface
Of course. Let $H_1 \subset \mathbb{P}^3$ be a smooth quadric surface and $H_2$ a tangent plane to $H_1$. Then $H_1 \cap H_2$ is a pair of intersecting lines. See Roy's answer below. 
Nov 21 
revised 
Computing the nonsingular projective model of a plane curve
edited body 
Nov 21 
answered  Computing the nonsingular projective model of a plane curve 
Nov 17 
revised 
When is a surface in a threefold contractible to a curve?
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Nov 17 
comment 
When is a surface in a threefold contractible to a curve?
@ArtiePrendergastSmith: Right, my answer only considers extremal contractions. I corrected it. Thanks for the remark. 
Nov 15 
comment 
Reference request for cohomology of coverings
But why are you sure that this is true in such a generality? 
Nov 15 
awarded  Enlightened 
Nov 15 
awarded  Nice Answer 
Nov 15 
revised 
Curves on varieties, and a criterion for nef divisor
improved formatting 
Nov 12 
comment 
Two (other) rings…are they isomorphic?
Anyway, already for isolated singularities, it is not true in general that the analytic (or formal) type of the singularity is the same of the type of the tangent cone. For instance, take $$\mathbb{C}[[x, y, z, w]]/(x^3+y^3+z^3+w^3), \quad \mathbb{C}[[x, y, z, w]]/(x^3+y^3+z^3+w^3+xyzw).$$ Then these two germs are not isomorphic: in fact they have the same Milnor number ($16$) but different Tjurina number ($16$ and $15$, respectively). 
Nov 12 
comment 
Two (other) rings…are they isomorphic?
Just a remark: in this case, the singularities of your hypersurfaces at the origin are not isolated. 
Nov 12 
comment 
Does some square of the first Chern class preserved by conifold transition?
See CortiSmith: Conifold transitions and Mori Theory, Corollary 2. intlpress.com/site/pub/pages/journals/items/mrl/content/vols/… 
Nov 12 
comment 
Two rings…are they isomorphic?
I completely agree with Vladimir Dotsenko. 
Nov 7 
revised 
Embedding proper algebraic spaces
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Nov 7 
revised 
Embedding proper algebraic spaces
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Nov 7 
answered  Embedding proper algebraic spaces 
Nov 7 
comment 
Embedding proper algebraic spaces
Are you sure that this exclude the possibility of embedding $S$ into a smooth algebraic space? It seems to me that this argument only works if $X$ is a scheme, or I am missing something? 
Nov 7 
awarded  Nice Answer 
Nov 7 
revised 
When is a surface in a threefold contractible to a curve?
added 145 characters in body 