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►We call the increasing solution for which the principal branch and denote it by .
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►►►Figure 4.13.1: Branches
, of the Lambert -function.
Magnify
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►Other solutions of (4.13.1) are otherbranches of .
…The otherbranches
are single-valued analytic functions on , have a logarithmic branch point at , and, in the case , have a square root branch point at respectively.
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►If and , then one branch is , the otherbranch is , with in both cases.
Similarly if , then one branch is , the otherbranch is , with in both cases.
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►(b) By specifying the value of at a point (not a branch point), and requiring to be continuous on any path that begins at and does not pass through any branch points or other singularities of .
►If the path circles a branch point at , times in the positive sense, and returns to without encircling any otherbranch point, then its value is denoted conventionally as .
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►again with analytic continuation for other values of , and with the principal branch defined in a similar way.
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►The same is true of otherbranches, provided that , , and are excluded.
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►For further information, including uniform asymptotic expansions, extensions to otherbranches of the functions and their derivatives, and extensions to half-integer values of the order, see Olver (1954).
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►The expansion (31.11.1) for a Heun function that is associated with any branch of (31.11.2)—other than a multiple of the right-hand side of (31.11.12)—is convergent inside the ellipse .
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