Beta-function recurrence 2026-09-28
The Beta function obeys
These identities follow immediately from the beta--gamma identity and the Gamma function recurrence.
With and formal edge conditions , the lubrication theory equation is . Its positive separated long-time profile has
Writing in terms of the beta function, the centre height is . The edge region of a draining viscous film determines how the singular outer profile matches the flow over the physical edge.
For and a fixed branch away from the cut,
Expand for and integrate termwise using the beta function to recover the Gauss hypergeometric function series. Analytic continuation extends the identity in .
In units , velocity integration of gives . The beta function separates the radial-speed and angular integrals. This constant is positive for every .
As , . Therefore
Thus every positive solution is unbounded when κ ≥ 8, including the logarithmically divergent critical case. For , by contrast,
For example the substitution identifies this normalizing constant as , with both beta function parameters positive.