In mathematics, the j-invariant, regarded as a function of a complex variable τ, is a modular function defined on the upper half plane of complex numbers with positive imaginary part. We can express it in terms of Jacobi's theta functions, in which form it can very rapidly be computed. We have

The numerator and denominator above are in terms of

and

These have the properties that

and possess the analytic properties making them modular forms. Δ is a modular form of weight twelve by the above, and g₂ one of weight four, so that its third power is also of weight twelve. The quotient is therefore a modular function of weight zero; this means j has the absolutely invariant property that

The fundamental region

The two transformations and together generate a group called the modular group, which we may identify with the projective linear group . By a suitable choice of transformation belonging to this group, , with ad-bc=1, we may reduce τ to a value giving the same value for j, and lying in the fundamental region for j, which consists of values for τ satisfying the conditions

As a Riemann surface, this has genus 0, and every (level one) modular function is a rational function in j; in other words the field of modular functions is .

The values of j are in a one-to-one relationship with values of τ lying in the fundamental region, and each value for j corresponds to the field of elliptic functions with periods 1 and τ, for the corresponding value of τ; this means that j is in a one-to-one relationship with isomorphism classes of elliptic curves.

Class field theory and j

The j-invariant has many remarkable properties. One of these is that if τ is any element of an imaginary quadratic field with positive imaginary part (so that j is defined) then j(τ) is an algebraic integer. The field extension is abelian, meaning with abelian Galois group. We have a lattice in the complex plane defined by 1 and τ, and it is easy to see that all of the elements of the field which send lattice points to other lattice points under multiplication form a ring with units, called an order. The other lattices with generators 1 and τ' associated in like manner to the same order define the algebraic conjugates j(τ') of j(τ) over . The unique maximal order under inclusion of is the ring of algebraic integers of , and values of τ having it as its associated order lead to unramified extensions of . These classical results are the starting point for the theory of complex multiplication.

The q-series and moonshine

Another remarkable property of j has to do with what is called its ‘’q-series’’. If we fix the imaginary part of τ and vary the real part, we obtain a periodic complex function of a real variable with period one. The Fourier coefficients for these functions are extremely interesting. If we perform the substitution q = exp(2πiτ) the Fourier series becomes a Laurent series in q, , where the values for c_n for n < -1 are all zero, and where the c_n are integers. The first few terms of it are

as we may easily find by substituting q for exp(2πiτ) in the definition for j with which we started. The coefficients c_n for the positive exponents of q are the dimensions of the grade n part of an infinite dimensional graded algebra representation of the Monster group called the moonshine module, a fact which may be taken as the starting point for moonshine theory.

Still another remarkable property of the q-series for j is the product formula; if p and q are small enough we have

Algebraic definition

So far we have been considering j as a function of a complex variable. However, as an invariant for isomorphism classes of elliptic curves, it can be defined purely algebraically. Let

y² + a₁xy + a₃y = x³ + a₂x² + a₄x + a₆

be a plane elliptic curve in any field of characteristic neither 2 nor 3 in which the coefficients lie. Then we may define

The j-invariant for the elliptic curve may now be defined as