Hello!
I've stumbled a couple of times on this forum while doing some research for my scientific project, which will most likely by the subject of my PhD thesis. Long story short - I want to develop a 4D representation for acoustical directivity by modelling time or frequency as 4th physical dimension and I need some 4D basis functions. As far as I know, the only ones used in engineering are hyperspherical harmonics, which I'm certainly going to use, but I was wondering whether or not I can find something else. Since I don't really need the extra dimension to be angular (time or frequency seem to be more of a linear dimension), I was thinking about some basis defined in spherindrical coordinates. This seems to be the only place in the internet that actually knows or cares about spherindrical coordinate system, so here's my question to you - do you know of any spherindrical basis?