[MUSIC] So now I would like to give you the answer on the question on Jacobi forms. Of singular weight. For the latest D8. Using the method which I would like to describe, you can solve this equation not only for D8, but for any root lattice, Dm. By definition Dm, this is the lattice. The maximum even sub lattice of the Euclidean lattice zET m, the lattice of all vectors, X1. XM, Zed m with even sums, with even sum of [INAUDIBLE] A simple exercise to find The lattice, modular DM means the discriminant group of this lattice. This discriminant group contains four element. 0, the vector E1, plus minus E1 + E2 plus Over 2. Here I used the standard basis of the lattice Zed m. Or in accordance, I can write it using the same as follows. 0, 1, 0, 1 and then + -1, 1 and so on. 1 over 2. So we have four vectors. I denote them as h0, h1, h2 for plus and h3 for minus. Now we have to calculate the matrix U(T) in the presentation, this matrix is very simple. This is diagonal. E phi i (h,h) for all h in the discriminate group. In our case, this is a falling diagonal matrix. 1,-1,1,1. This is the case of the lattice D8. For the lattice u(s), the calculation is a little bit more complicated. So we have here one-half Then in the first line, we have for 1 in the first column. For 1, we have 1 units on the main diagonal and -1 in all other places. And we're looking for common eigenvectors of these two matrices. Then I have common eigenvectors. The first one, 1, 0, 1, 0. The second, 0, 0, 1, -1. It follows then we have two Jacobi forms of singular weight for the lattice D8. The first function. Corresponds to the vector D1. This is the Jacobi theta series with characteristic h0, characteristic 0 + The Jacobi theta series with characteristic h2 corresponds to these two units. The second function, the function psi for the lattice D8, we can get this function taking the second vector. Have two [INAUDIBLE] coordinate, so we have theta D8H h2- theta D8, h3. Now we have to compare this function with two examples we constructed before. So we have two function. The Fourier coefficient of the second function starts with one and for the second function, we start with this trivial coefficient zero. Analyzing the first Fourier coefficient, you see that the first formula gives us the Jacobi theta series for the lattice E8. And the second, The function D8. You have to check only the signature here in principle. But I hope that the signature as a sign plus is correct in this formula. So you see, using the same method, you can analyze any lattice. So we'll have a lattice, we have to find the matrices of the wider presentation for the two generators of the group and then we have to find the common vectors. But you can have common vectors with different eigenvectors. It means that in some cases, you get Jacobi modular forms of singular weight with some character. We had this ethic for the latest A1. Second example, the lattice A1 or this is read as Eichler- Zegie Jacobi series is exactly Jacobi form of single weight one-half, because the rank of this lattice is equal to one one-half, but also the index one-half, with multiply system and a character of the Heisenberg group. So, this is Jacobi form of single weight but with non character. In one variable for the full modular group, we have the second function, which is our notation. With Eichler we use this notation. Theta is three-half. This is Jacobi form of weight one-half and index three-half. With the falling multiply system of 4 the 24, and the formula was for this, so three-halfs to zeta. This is theta times theta to zeta over theta zeta. This is the so called Quintuple Product. It's very easy to prove that these function are the only Jacobi forms of singular weight for the full and modular group. This is what we know about the singular weight. But, what about the next weight? [SOUND] So, we fix the lattice. L of rank n0. Then, the minimal possible weight k = n0 over 2 is singular. The next weight, this is n zero plus over 2, plus one half. This weight is called critical. This is the next possible weight. And this is the first possible weight for parabolic Jacobi form, because for the case of single weight Jacobi modular form. This is the sum with some constant coefficient ch. Of the Jacobic theta series. It follows that a Fourier coefficient of this function, l in the lattice, is not 0, then 2n minus l to the square is equal to 0. This is the main property of Jacobi form of single weight. One can say that the Jacobi form of single weight is a generalization of a constant. If we consider a constant as modular form of weight zero, the hyperbolic index of all Fourier coefficient of Fourier. Modular form of single weight, the hyperbolic is equal to 0. So this function is never parabolic. Or, it's better to say this function is anti-cusp, anti-parabolic. But the critical weight is the first possible weight where a Jacobi cast form could exist. But in the case of Heigler zageer, we have the following remarkable theorem of Nils Skoruppa is true. This now classical result of Nils Skoruppa. He proved this, I think, in '86 in his PhD. And he proved that the space of Jacobi form of weight 1 and index m, this is the case of Eichler Zegier is always trade though. Therefore there are no Jacobi forms of critical weight in the case of Eichler Zegier. And what about the case of Jacobi form in many variables? [SOUND]
