[MUSIC] I would like to add one exercise which is not so simple. Exercise. Question. The function which we construct, is it the first cusp form of weight 4 and index 7? I can formulate it in the following way. If you consider the space of Jacobi cusp form of weight, four, in index seven, index six, this space is trivial or not? Certainly the same equation we can put in our index five, four, three, and so on. So this function, repeat, this function is a constructor. This is a first cusp form of weight for 4 or not. The method which we discussed today is not only a series of examples. Using this approach, we can prove some theorem. Now, I would like to prove the following theorem. As a dimension of the space of Jacobi form of odd weight, any odd weight, and index 2 is equal to the dimension of the space of usual modular cusp form of weight 2 k + 2. So I can formulate it as follows. As linear space. These spaces are isomorphic. Moreover, in the proof of the theorem, we can construct this isomorphism explicitly. As a proof, I will use the idea of theta products or theta blocks as a proof. First of all, I would like to analyze the divisor of any Jacobi form of odd weight. The divisor of a holomorphic Jacobi form of weight, 2k + 1 and index M always contains four points, modular, the standard latest, zed tau plus zed 0 one-half tau over 2 tau plus 1 over 2 modular zed tau + zed. I can write down the same fact in the following simpler way. The divisor of the corresponding Jacobi form contains the latest one-half zed Tao plus zed. Okay, we can prove this using the functional equation of Jacobi form. First of all, the value of this Jacobi form in 0 is a modular form of odd weight with respect to the full modular group. Certainly, the space is trivial. This is the first 0 of the corresponding Jacobi form. Now. Let me calculate its value. At point, Tau over 2 minus tau. Using the elliptic equation of our Jacobi form, we get e to the power -2 pi i m. Then, lambda to the square tau but lambda is equal to -1 here, so, we have tau. Minus, because lambda is equal to minus one two times zed. But zed is tao over two fe to the power two k plus one m Tau, tau over two. So, I used only the elliptic equation. This term, corresponding factor, certainly is one. But, tau over two minus tau Is minus tau over 2. And this is -1 to the weight of our function Any Jacobi form of odd weight is odd function. So, this is minus one. We have proved that this function Vanishes and tau over 2. Exactly the same proof The same proof using the elliptic equation, we can realize for any other points of order two, so for one half and tao plus 2 over 2. So we'll prove this very important property. Then the divisor of any Jacobi form of odd weight contains at least four points. In particular, I can fix this [INAUDIBLE]. This fact. This space of Jacobi form of odd weight in index 1 is 3. Because any Jacobi form Holomorphic or weak, it doesn't matter. Any function of index one has only two zeroes in any fundamental domain of our latest. But arbitrary Jacobi form of odd weight has at least four zeros. Now, I would like to analyze the Jacobi form of weight of index two, sorry. To construct all Jacobi forms of index two and odd weight, I would like to take the following theta block. The Jacobi form, weight form, of weight -1 and index 2. We consider it this function as the last lecture. This is the product, the quotient of theta function over eta q tau. And this function has exactly, has this divisor. [INAUDIBLE] in four points of the fundamental domain because the divisor of this function is equal to the divisor of theta 2 zed. This is equal exactly 1/2. And now, let me analyze in arbitrary Jacobi form of index two, and an odd weight. The divisor of this function contains this divisor and exactly equal to it. Because, we know, that the Jacobi form of weight two has exactly four divisors. Follows when this function is holomorphic and weak. Holomorphic function in this weak Jacobi form of weight 2 k plus 2 and index zero. Why is Jacobi form is a weak form, because we know the Fourier expansion of this function. The Fourier expansion starts by r minus r to the power minus one plus q. So this quotient contains only non negative powers of q in this fourier expansion so this function is weak but this space of weak Jacobi form of weight 2k plus 2 is equal. To the space of usual modular form for the full modular group. So we see then our Jacobi form which we try to analyze is equal. Pi minus 1 2 times a modular form in one variable of weight 2k + 2. If, Pi is Eisenstein series, Eisenstein series of correspondent weight. And the Eisenstein series starts the Forier expansion of Eisenstein series starts by 1. Then, phi- 2 times Eisenstein series is not holomorphic because it starts for the expansion by q something. And the index, the hyperbolic norm of the correspondent index. Is equal to 4 times 0 times the integer Jacobi form minus 1 to the square, is equal to -1. So certainly, this Jacobi form is not holomorphic. But if we multiply this function by cusp form of [INAUDIBLE] 2k+2 cusp form. Then we can take [INAUDIBLE] delta function here as a vector. And this function is our function, phi 11, 12, which is holomorphic. Holomorphic L infinity. So, we proved that in this case, we get a function in J 2k+1, 2. [MUSIC]
