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23 Weierstrass Elliptic and Modular FunctionsWeierstrass Elliptic Functions

§23.10 Addition Theorems and Other Identities

Contents
  1. §23.10(i) Addition Theorems
  2. §23.10(ii) Duplication Formulas
  3. §23.10(iii) n-Tuple Formulas
  4. §23.10(iv) Homogeneity

§23.10(i) Addition Theorems

23.10.1 ℘⁡(u+v)=14⁢(℘′⁡(u)−℘′⁡(v)℘⁡(u)−℘⁡(v))2−℘⁡(u)−℘⁡(v),
23.10.2 ζ⁡(u+v)=ζ⁡(u)+ζ⁡(v)+12⁢ζ′′⁡(u)−ζ′′⁡(v)ζ′⁡(u)−ζ′⁡(v),
23.10.3 σ⁡(u+v)⁢σ⁡(u−v)σ2⁡(u)⁢σ2⁡(v)=℘⁡(v)−℘⁡(u),
23.10.4 σ⁡(u+v)⁢σ⁡(u−v)⁢σ⁡(x+y)⁢σ⁡(x−y)+σ⁡(v+x)⁢σ⁡(v−x)⁢σ⁡(u+y)⁢σ⁡(u−y)+σ⁡(x+u)⁢σ⁡(x−u)⁢σ⁡(v+y)⁢σ⁡(v−y)=0.

For further addition-type identities for the σ-function see Lawden (1989, §6.4).

If u+v+w=0, then

23.10.5 |1℘⁡(u)℘′⁡(u)1℘⁡(v)℘′⁡(v)1℘⁡(w)℘′⁡(w)|=0,

and

23.10.6 (ζ⁡(u)+ζ⁡(v)+ζ⁡(w))2+ζ′⁡(u)+ζ′⁡(v)+ζ′⁡(w)=0.

§23.10(ii) Duplication Formulas

23.10.7 ℘⁡(2⁢z)=−2⁢℘⁡(z)+14⁢(℘′′⁡(z)℘′⁡(z))2,
23.10.8 (℘⁡(2⁢z)−e1⁡)⁢℘′2⁢(z)=((℘⁡(z)−e1⁡)2−(e1⁡−e2⁡)⁢(e1⁡−e3⁡))2.

(23.10.8) continues to hold when e1⁡, e2⁡, e3⁡ are permuted cyclically.

23.10.9 ζ⁡(2⁢z)=2⁢ζ⁡(z)+12⁢ζ′′′⁡(z)ζ′′⁡(z),
23.10.10 σ⁡(2⁢z)=−℘′⁡(z)⁢σ4⁡(z).

§23.10(iii) n-Tuple Formulas

For n=2,3,…,

23.10.11 n2⁢℘⁡(n⁢z)=∑j=0n−1∑ℓ=0n−1℘⁡(z+2⁢jn⁢ω1+2⁢ℓn⁢ω3),
23.10.12 n⁢ζ⁡(n⁢z)=−n⁢(n−1)⁢(η1+η3)+∑j=0n−1∑ℓ=0n−1ζ⁡(z+2⁢jn⁢ω1+2⁢ℓn⁢ω3),
23.10.13 σ⁡(n⁢z)=An⁢e−n⁢(n−1)⁢(η1+η3)⁢z⁢∏j=0n−1∏ℓ=0n−1σ⁡(z+2⁢jn⁢ω1+2⁢ℓn⁢ω3),

where

23.10.14 An=n⁢∏j=0n−1∏ℓ=0ℓ≠jn−11σ⁡((2⁢j⁢ω1+2⁢ℓ⁢ω3)/n).

Equivalently,

23.10.15 An=(π2⁢G2ω1)n2−1⁢qn⁢(n−1)/2in−1⁢exp⁡(−(n−1)⁢η13⁢ω1⁢((2⁢n−1)⁢(ω12+ω32)+3⁢(n−1)⁢ω1⁢ω3)),

§23.10(iv) Homogeneity

For any nonzero real or complex constant c

23.10.17 ℘⁡(c⁢z|c⁢𝕃) =c−2⁢℘⁡(z|𝕃),
23.10.18 ζ⁡(c⁢z|c⁢𝕃) =c−1⁢ζ⁡(z|𝕃),
23.10.19 σ⁡(c⁢z|c⁢𝕃) =c⁢σ⁡(z|𝕃).

Also, when 𝕃 is replaced by c⁢𝕃 the lattice invariants g2⁡ and g3⁡ are divided by c4 and c6, respectively.

For these results and further identities see Lawden (1989, §6.6) and Apostol (1990, p. 14).