>>100
u･∫[0,2π]dθ∫[0,∞] exp(-k･r) r･dr
=u･2π･{[r･exp(-kr)-∫(-exp(-k･r)/k dr}_[0,∞]
=u･2π･{(1/k)∫[0,∞]exp(-k･r)dr}
=u･2π･(1/k){(-1/k)exp(-k･r)}
=u･2π/(k^2)