Digital Library of Mathematical Functions
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10 Bessel FunctionsBessel and Hankel Functions

§10.18 Modulus and Phase Functions

Contents

§10.18(i) Definitions

For \nu\geq 0 and x>0

where \mathop{M_{{\nu}}\/}\nolimits\!\left(x\right) (>0), \mathop{N_{{\nu}}\/}\nolimits\!\left(x\right) (>0), \mathop{\theta_{{\nu}}\/}\nolimits\!\left(x\right), and \mathop{\phi_{{\nu}}\/}\nolimits\!\left(x\right) are continuous real functions of \nu and x, with the branches of \mathop{\theta_{{\nu}}\/}\nolimits\!\left(x\right) and \mathop{\phi_{{\nu}}\/}\nolimits\!\left(x\right) fixed by

10.18.3
\mathop{\theta_{{\nu}}\/}\nolimits\!\left(x\right)\to-\tfrac{1}{2}\pi,
\mathop{\phi_{{\nu}}\/}\nolimits\!\left(x\right)\to\tfrac{1}{2}\pi,x\to 0+.

§10.18(ii) Basic Properties

10.18.13x^{2}{\mathop{M_{{\nu}}\/}\nolimits^{{\prime\prime}}}\!\left(x\right)+x{%
\mathop{M_{{\nu}}\/}\nolimits^{{\prime}}}\!\left(x\right)+(x^{2}-\nu^{2})%
\mathop{M_{{\nu}}\/}\nolimits\!\left(x\right)=\frac{4}{\pi^{2}{{\mathop{M_{{%
\nu}}\/}\nolimits^{{3}}}(x)}},
10.18.15x^{3}w^{{\prime\prime\prime}}+x(4x^{2}+1-4\nu^{2})w^{{\prime}}+(4\nu^{2}-1)w=0,w=x{\mathop{M_{{\nu}}\/}\nolimits^{{2}}}\!\left(x\right).
10.18.16{{\mathop{\theta_{{\nu}}\/}\nolimits^{{\prime}}}^{{2}}}\!\left(x\right)+\frac{%
1}{2}\frac{{\mathop{\theta_{{\nu}}\/}\nolimits^{{\prime\prime\prime}}}\!\left(%
x\right)}{{\mathop{\theta_{{\nu}}\/}\nolimits^{{\prime}}}\!\left(x\right)}-%
\frac{3}{4}\left(\frac{{\mathop{\theta_{{\nu}}\/}\nolimits^{{\prime\prime}}}\!%
\left(x\right)}{{\mathop{\theta_{{\nu}}\/}\nolimits^{{\prime}}}\!\left(x\right%
)}\right)^{2}=1-\frac{\nu^{2}-\tfrac{1}{4}}{x^{2}}.

§10.18(iii) Asymptotic Expansions for Large Argument

As x\to\infty, with \nu fixed and \mu=4\nu^{2},

The remainder after k terms in (10.18.17) does not exceed the (k+1)th term in absolute value and is of the same sign, provided that k>\nu-\tfrac{1}{2}.