Thom Tyrrell
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 Jun 9 comment Better understanding regular functions on a Projective variety One small detail - $\phi$ is regular only on a subset of $W$. What are $u$ and $h$? They can be solved for with linear algebra. Jun 2 comment Hasse invariant of quaternions over $\mathbb{Q}_p$ When you say "the" quaternion algebra over $\mathbb{Q}_p$, do you mean Hamilton's $(-1,-1)$ quaternion algebra? There are two quaternion algebras over $\mathbb{Q}_p$ up to isomorphism. Jan 14 comment What are normal schemes intuitively? If $(\sqrt{-3}-1)^2 \equiv 0$ (mod 2) and 2 is prime in this ring, then what is $\sqrt{-3}-1$ congruent to mod 2? And how would the norms of these numbers be related? Sep 14 comment A question about hyperelliptic curve I'm a bit confused about (i) and (ii). In (i), could we not take $A=1$ and $B=0$? And if $t$ is transcendental over $k$, wouldn't $k[t]^* \neq k^*$? Mar 22 comment Zeta Function of a Curve You may find these lecture notes interesting: math.lsa.umich.edu/~mmustata/lecture3.pdf. Have you read Silverman's book on elliptic curves? It contains a section on zeta functions. Mar 14 comment $a^4 = x^2 + y^2 (6+8b)$ solutions $(x,y)$? The lines that pass through $(a^2,0)$ are of the form $x = my + a^2$, where $m$ is a rational number. Substituting for $x$ into the main equation, we can solve for $y$ in terms of $m$, and likewise for $x$. For what values of $m$ are the resulting values of $x,y$ integral? Aug 26 comment Kähler differentials of a hyperelliptic curve. The ds and dZ play a role in the restriction/transition maps. Aug 26 comment Kähler differentials of a hyperelliptic curve. With the restriction maps you can explicitly take a section on U' and a germ at $(0,0)$, and produce a family of sections on neighborhoods of $(0,0)$. The map from $\Omega^1_{V'/V}$ to the stalk at $(0,0)$ sends a polynomial to its constant term. So, we can extend germs (constants) to open nbhds as the constant terms of polynomials in $s$ with the higher order terms coming from the section on $U'$ restricted to $U' \cap V'$. Aug 26 comment Kähler differentials of a hyperelliptic curve. If you work out the restriction maps it might make the isomorphism more clear. In the odd case, with the decomposition $X = U' \cup \{(s,z) = (0,0)\}$, we can give global sections by gluing sections from the open cover of U' plus a basis around $(0,0)$. A global section is then a relative differential on $U'$, and a collection of differentials on open nbhds of $(0,0)$ - i.e. a germ. Perhaps the "as" is misleading, but there should be a slick way to see the direct sum decomposition. Aug 26 comment Kähler differentials of a hyperelliptic curve. What is the restriction map from $\Omega^1_{V'/V}$ to $U' \cap V'$? When the degree of $P$ is odd $\Omega^1_{U'/U}$ and $\Omega^1_{V'/V}$ have different dimensions over k (d versus d+1), so when you glue $U'$ and $V'$ to construct a global relative differential, this is where that extra dimension comes from - the single point over $s=0$. Aug 26 comment Prove $X^2+Y^2-1$ is irreducible using geometrical tools. Indeed. I've read your answer and I will edit mine. Aug 26 comment Prove $X^2+Y^2-1$ is irreducible using geometrical tools. In terms of style, the author may simply want to give a very general proof (one that applies when $k$ is not the real numbers). We also don't need to consider the whole infinite set of solutions to finish the proof, so the author might just be trying to keep things as simple as possible. It is not as important here that our field is infinite as that its characteristic is not 2. Aug 22 comment a diophantine equation from Stewart and Tall Have you tried reduction modulo a prime? Jul 23 comment Visualize a projective curve $X^3+Y^3=Z^3$ in $P_2(C)$ as a torus I would first replace y with y+1 and do some algebra to get the equation into Weierstrass form. Then, you can describe the torus with two patches. Jun 17 comment Gluing schemes Hartshorne example $\phi$ is the isomorphism we use to glue the schemes $X_1$ and $X_2$ together. The equality you are unable to interpret is the statement that the sections of our glued scheme X on an open set V are pairs $(s_1, s_2)$ which are equal when restricted to $U_1$ and $U_2$, respectively; we use $\phi$ to identify $U_1$ with $U_2$ in $X$. Feb 14 comment A Function Meromorphic with Poles at the Primes Can you construct a function with poles at the integers? Jan 30 comment Proving the condition for two elliptic curves given in Weierstrass form to be isomorphic There is a chapter devoted to it in Silverman's book. Jan 29 comment Proving the condition for two elliptic curves given in Weierstrass form to be isomorphic If you don't like this Riemann-Roch, function field business, have you read the main theorems concerning elliptic curves over the complex numbers? You can prove the same statement - all elliptic curves have a Weierstrass equation - using tools from analysis (eg. comparing coefficients of Laurent expansions). Jan 16 comment Properties of the Mandelbrot set, accessible without knowledge of topology? Why is this tagged as algebraic geometry? Jan 5 comment Question about the definition of weekly modular of weight k A lot is known about non-holomorphic modular forms (search for Maass Forms). Note that there is no trouble with the definition; if f has a pole at $\tau$, then $\gamma(\tau)$ must also be a pole as the factor $(cx+ d)^k$ is certainly holomorphic at $\tau$.