= Modular lambda function
{title2=$\lambda(\tau)=\theta[1/2,0](0,\tau)^4/\theta[0,0](0,\tau)^4$}
{wiki}
The nonzero even theta constants and the <Jacobi abstruse identity> give $\lambda(-1/\tau)=1-\lambda(\tau)$ and $\lambda(\tau+1)=\lambda(\tau)/(\lambda(\tau)-1)$. It is invariant under the principal <congruence subgroup> of level two. This parameter puts the associated complex <elliptic curve> in Legendre form $y^2=x(x-1)(x-\lambda)$; its <Klein j-invariant> is $256(1-\lambda+\lambda^2)^3/[\lambda^2(1-\lambda)^2]$.
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