|  Spherical Harmonics | 
Scalar Functions for S2
  * Definition: If
    Plm
    are the associated Legendre functions,
Ylm(θ, φ) = {[(2l + 1)/4π] [(l – m)!/(l + m)!]}1/2 eimφ Plm(cosθ) .
* Properties: Relationship between Ylm and Yl,−m, orthonormality and completeness,
Yl,−m = (−1)m Ylm* ; ∫ dΩ Y*l'm'Ylm = δll' δmm' ;
∑l=0∞ ∑m=−ll Y*lm(θ', φ') Ylm(θ, φ) = δ(φ−φ') δ(cosθ−cosθ') .
* Use in expansions: A useful formula is
| x−x' |−1 = 4π ∑l ∑m (2l+1)−1 (r< / r>l+1) Ylm*(θ', φ') Ylm(θ, φ) .
  @ General references: Schmid AJP(58)oct [explicit expressions, and group-representation theory];
    Pérez Saborid a0806/AJP [Maxwell-Thomson-Tait coordinate-free approach];
    Weitzman & Freericks CondMP(18)-a1805 [calculations without derivatives].
  @ Related topics:
    Coster & Hart AJP(91)apr [addition theorem];
    Ma & Yan a1203
      [rotationally invariant products of three spherical harmonics].
  > Online resources:
    Wikipedia page.
Generalizations and Other Spaces
  > s.a. manifolds [superspace]; multipoles.
  * Tensor spherical harmonics:
  * For S3:
    The eigenfunctions of L2, belonging
    to representations of SO(4), given by
ψnlm(α, θ, φ) = il Ylm(θ, φ) Ml−1 sinlα (dl+1cosα / d(cosα)l+1) ,
Ml = [n2 (n2 − 12) ··· (n2 − l2)]1/2 .
  @ Vector spherical harmonics:
    Hill AJP(54)apr;
    Novitsky a0803 [and Maxwell theory].
  @ On S3:
    Fock ZP(35);
    in Lifshitz & Khalatnikov AiP(63);
    Meremianin JPA(06)mp/05;
    Lindblom et al GRG(17)-a1709 [scalar, vector and tensor harmonics].
  @  Tensor spherical harmonics: Mandrilli et al GRG(20) [correspondence between tensorial spin-s and spin-weighted].
  @ Spin-weighted: Scanio AJP(77)feb [and electromagnetic fields];
    Straumann a1403
      [as vector-valued functions on the total space SO(3) of the Hopf bundle];
    Shah & Whiting GRG(16)-a1503 [spin-weighted spheroidal harmonics, raising and lowering operators];
    Boyle JMP(16)-a1604 [geometry and definition].
  @ In higher dimensions:
    Frye & Efthimiou a1205;
    Gundlach et al CQG(13) [for the wave equation, summation-by-parts methods].
  @ In superspace: Zhang & Zou JMP(05)m.RT/06 [homogeneous superspaces];
    De Bie & Sommen JPA(07)-a0705 [and integration].
  @ Related topics: Dolginov JETP(56) [pseudo-euclidean];
    Hughes JMP(94) [higher spin];
    Ramgoolam NPB(01) [fuzzy spheres];
    Coelho & Amaral JPA(02)gq/01 [conical spaces];
    Mweene qp/02;
    Cotăescu & Visinescu MPLA(04)ht/03 [euclidean Taub-NUT];
    Mulindwa & Mweene qp/05 [l = 2];
    Hunter & Emami-Razavi qp/05/JPA [fermionic, half-integer l and m];
    Bouzas JPA(11), JPA(11) [spin spherical harmonics, addition theorems];
    Alessio & Arzano a1901 [non-commutative deformation].
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