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14 Legendre and Related FunctionsComplex Arguments

§14.27 Zeros

P^{\mu}_{\nu}\left(x\pm i0\right) (either side of the cut) has exactly one zero in the interval (-\infty,-1) if either of the following sets of conditions holds:

  • (a)

    \mu<0, \mu\notin\mathbb{Z}, \nu\in\mathbb{Z}, and \sin\left((\mu-\nu)\pi\right) and \sin\left(\mu\pi\right) have opposite signs.

  • (b)

    \mu,\nu\in\mathbb{Z}, \mu+\nu<0, and \nu is odd.

For all other values of the parameters P^{\mu}_{\nu}\left(x\pm i0\right) has no zeros in the interval (-\infty,-1).

For complex zeros of P^{\mu}_{\nu}\left(z\right) see Hobson (1931, §§233, 234, and 238).