- Hasse-Witt matrix
In
mathematics , the Hasse-Witt matrix "H" of anon-singular algebraic curve "C" over afinite field "F" is the matrix of theFrobenius mapping ("p"-th power mapping where "F" has "q" elements, "q" a power of theprime number "p") with respect to a basis for thedifferentials of the first kind . It is a "g" × "g" matrix where "C" has genus "g".Approach to the definition
This definition, as given in the introduction, is natural in classical terms, and is due to
Helmut Hasse andErnst Witt (1936 ). It provides a solution to the question of the "p"-rank of theJacobian variety "J" of "C"; namely, that the "p"-rank is the same as the rank of "H". It is also a definition that is in principle algorithmic. There has been substantial recent interest in this as of practical application tocryptography , in the case of "C" ahyperelliptic curve . The curve "C" is superspecial if "H" = 0.That definition needs a couple of caveats, at least. Firstly, there is a convention about Frobenius mappings, and under the modern understanding what is required for "H" is the "transpose" of Frobenius (see
arithmetic and geometric Frobenius for more discussion). Secondly, the Frobenius mapping is not "F"-linear; it is linear over theprime field Z/"p"Z in "F". Therefore the matrix can be written down but does not represent a linear mapping in the straightforward sense.Cohomology
The interpretation for
sheaf cohomology is this: the "p"-power map acts on:"H"1("C","O""C"),
or in other words the first cohomology of "C" with coefficients in its
structure sheaf . This is now called the Cartier-Manin operator (sometimes just Cartier operator), for Pierre Cartier andYuri Manin . The connection with the Hasse-Witt definition is by means ofSerre duality , which for a curve relates that group to:"H"0("C", Ω"C")
where Ω"C" = Ω1"C" is the sheaf of
Kähler differential s on "C".Abelian varieties and their "p"-rank
The "p"-rank of an
abelian variety "A" over a field "K" ofcharacteristic p is the integer "k" for which the kernel "A" ["p"] of multiplication by "p" has "p""k" points. It may take any value from 0 to "d", the dimension of "A"; by contrast for any other prime number "l" there are "l"2"d" points in "A" ["l"] . The reason that the "p"-rank is lower is that multiplication by "p" on "A" is aninseparable isogeny : the differential is "p" which is 0 in "K". By looking at the kernel as agroup scheme one can get the more complete structure (referenceDavid Mumford "Abelian Varieties" pp.146-7); but if for example one looks atreduction mod p of adivision equation , the number of solutions must drop.The rank of the Cartier-Manin operator, or Hasse-Witt matrix, therefore gives a formula for the "p"-rank. In the original paper of Hasse and Witt the problem is phrased in terms intrinsic to "C", not relying on "J". It is there a question of classifying the possible
Artin-Schreier extension s of thefunction field "F"("C") (the analogue in this case ofKummer theory ).Case of genus 1
The case of
elliptic curve s was worked out by Hasse in 1934. There are two cases: "p"-rank 0, or supersingular elliptic curve, with "H" = 0; and "p"-rank 1, ordinary elliptic curve, with "H" ≠ 0. Here there is a congruence formula saying that "H" is congruent modulo "p" to the number "N" of points on "C" over "F", at least when "q" = "p". Because ofHasse's theorem on elliptic curves , knowing "N" modulo "p" determines "N" for "p" ≥ 5. This connection withlocal zeta-function s has been investigated in depth. (Supersingular curves are related tosupersingular prime s, but by means ofmodular curve s and particular points on them.)References
*Hasse, Helmut, "Existenz separabler zyklischer unverzweigter Erweiterungskörper vom Primzahlgrad p über elliptischen Funktionenkörpern der Charakteristik p", Journal f. d. reine u. angew. Math. 172 (1934), 77-85.
*Hasse, Helmut & Witt, Ernst, "Zyklische unverzweigte Erweiterungskörper vom Primzahlgrad p über einem algebraischen Funktionenkörper der Charakteristik p", Monatshefte f. Math. und Phys. 43 (1936), 477-492
*Ju. I. Manin. "The Hasse-Witt matrix of an algebraic curve." Trans. Amer. Math. Soc., 45:245-246, 1965 (English translation of a Russian original)
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