ELLIPTIC MODULAR FUNCTIONS 7
forms containing A is one; i.e., given any other 8x8 positive-definite,
symmetric, integral matrix Β with determinant 1 and even diagonal
entries, there exists a non-singular, unimodular, integral 8x8 matrix
Μ such that 'MAM=B, where 'M denotes the transpose of M. If
x = (xv ···, xs) is an 8-tuple of numbers, let A[x] = *xAx, where χ is to
be viewed as a column vector. Let Ζ denote the rational integers.
If 2 e D, the following series
U*)= Σ e(-LzA[»])
n = (tij, ,.,)621 \ 2 I
converges uniformly on compact subsets of D. Since the diagonal
entries of A are even, θΑ(ζ+1) = θΑ(ζ). Since A is unimodular and
A = AA~1A, A'1 is unimodularly equivalent to A. Using the Poisson
summation formula (to be proved later: see Chapter 13, section 1) one
then sees that 0( — ζ~1) = ζ*θ(ζ). Since T: z^z+1 and S: z-+— z~l gen
erate Γ, it is now clear that 0eM2. Since dim M2=l, we have 0 = cE2.
Comparing the constant terms in the Fourier expansions one obtains
c = l, so that Θ = ΕΤ Thus one obtains the interesting (classical) result
that for any positive integer, the number vm of ways of representing
2m in the fo.rm 2m=A[ii], n<=Z\ is equal to 240 times the number
theoretic function Σ dl. (If we use A as the metric form on the space
dim
of root vectors for £78, the roots themselves acquire length two.) By
expressing the Fourier coefficients of E2 in another way, one also
obtains (as we shall later in a more general situation Chapter 12)
that vm is the product of the p-adic densities of representations of
2m by A, taken over all primes ρ and oo. Thus one obtains a special
case of Siegel's main formula for definite quadratic forms [56b]. It
is expressed in this case, and characteristically is given, by an iden
tity between a 0-series whose Fourier coefficients give global average
densities and an Eisenstein series whose Fourier coefficients are Euler
products of local densities.
Another question of importance is to find a common denominator
for the Fourier coefficients of Eg. Since, by (7), Bgan° <= Z, the denomi
nators are bounded for any fixed g and, in any case, are closely con
nected with irregular primes [10; 33; 42; 56f]. A similar phenomenon
occurs for G = PSp(n, R), r = PSp(n, Z), D={Z=X+iY\ where Ζ is
nxn, 'Z—Z, Y> 0}, and it may be conjectured that the phenomenon is
rather general (cf. [35] and [58]).