There are (at least) two equations known as Sommerfeld's formula. The first is
where is a Bessel function of the first kind. The second states that under appropriate restrictions,
There are (at least) two equations known as Sommerfeld's formula. The first is
where is a Bessel function of the first kind. The second states that under appropriate restrictions,
Weisstein, Eric W. "Sommerfeld's Formula." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/SommerfeldsFormula.html