> > > >

It!. m(t, s,O') dO' +1" It!. n(t,s,r,O')dO'dr. Putting now (13.15) into (13.7) - (13.10) we obtain

f(t,s) = c(t,s) +l",(t,s,r)dr (13.16)

(13.17)

(13.18)

and

(13.19)