Open Letter to Ole L. Trinhammer and Henrik G. Bohr: The Integer 4 in πr_p = 2λ_p

Trinhammer and Bohr (EPL 128, 21001, 2019) derived the proton charge radius from the topology of a wavefunction on the compact configuration space U(3): πrp = 2λp, with λp the full Compton wavelength. Written with the reduced Compton wavelength ƛp = λp/2π, the same relation reads rp = 4ƛp — an integer that does not appear in their paper. This letter acknowledges their priority, explains how the integer was found independently in a discrete-lattice model, and asks three questions about its meaning in their framework. Open letter. Background: §5.1 and Appendix A5.1 of Theory of Space, Volume 1. https://theoryof.space