A substance diffuses in infinite space with initial concentration (r ) = 1

for r < a, and (r ) = 0 for r > a. Find a formula for the concentration

at later times. (Hint: It is radial. You can substitute v = ru to get a

problem on a half-line.)

Strauss, Walter PDEs

Issue, i know how to switch to a regular heat eq ut = kurr, but I dont know the BCs, only the ICs. So I'm having trouble solving it.