For , apply Jensen's formula with the smooth curvature term to the hyperbolic derivative density. Its distributional Laplacian is , giving the displayed identity with ramification points defined by . If at zero, use as the origin counting term and replace the base constant by . Without this regularization the basepoint-critical formula is undefined.
Past exam of the mathematics course of the University of Cambridge 2005 iii Paper 86 6 Solution Created 2026-10-03 Updated 2026-10-14
There are two necessary qualifications to the printed formula. Critical points satisfy , not . At a point with , the local degree is one and the printed summand is zero. In addition, is finite only when ; constant maps and origin-critical maps need separate treatment.
These are genuine issues even for otherwise ordinary disc maps. The map maps the unit disc into itself and has . Its derivative never equals one in the disc, so the printed count is zero, but it has a ramification point at of multiplicity one. For the correct identity has the additional term , proving that the displayed identity with the literal printed count is false. Also shows the undefined basepoint logarithm in an origin-critical example.
Prove the intended identity first for a nonconstant map with . Let be its hyperbolic derivative density and . Away from derivative zeros, is harmonic, and direct differentiation givesAt a zero of of multiplicity , factor with . Its logarithm contains , whose distributional Laplacian is . ThusThe local map degree is , so this is the required excess-degree weighting.
For a smooth function, differentiating its circular mean and applying the divergence theorem gives . Integrating in gives the logarithmic Green-Jensen formula. Factoring the isolated logarithmic singularities extends it to the present :Substitution yields the critical-point Jensen identity for the hyperbolic derivativewhere . This is the requested formula with the critical-point typo corrected. Initially choose a circle containing no ramification point on its boundary; continuity and integrability of logarithmic boundary singularities extend the result to the remaining radii.
By Schwarz-Pick theorem, . ThereforeThe normalized hyperbolic distance is , andConsequentlyOne can take and . Since the corrected count is nonnegative in the noncritical-basepoint case, this also proves the growth bound for with the literal printed, identically zero count, though not its false identity.
For completeness, if vanishes to order at zero, put andApply the same formula to , which is regular at zero. It givesThe origin contribution is a regularized Jensen term, not the undefined . If one instead omits the origin term, the upper bound gains ; this is bounded for , while the characteristic and off-origin count are bounded on any smaller compact disc. Thus a bound still holds for that nonnegative-count convention after enlarging . For a constant map and , so its characteristic growth bound is trivial; the logarithmic identity and a finite critical-divisor count are not defined. These qualifications resolve all maps allowed by the original opening sentence without asserting an undefined equality.