Do you know of anyone who has mathematically derived the 73 Ohms
of a dipole in free space?

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Anybody, such as an 18-year old Japanese student, can do it who can
integrate the power radiated by each elemental length of wire over a
surrounding sphere.

The radiation pattern comes out in the wash. It's an elementary matter.
Hopefully you are able to do it for yourself.

It is likely Heaviside was the first, mostly in his head, but because it was
so obvious he never bothered to write it down.

The Ph.D's of his age would have ridiculed the idea anyway on the grounds
that some of his other more important work lacked rigor. MFJ-259B's and
Viagra were yet to come.