  A new transform method for solving boundary value problems for linear and
integrable nonlinear PDEs recently introduced in the literature is used here to
obtain the solution of the modified Helmholtz equation
q_{xx}(x,y)+q_{yy}(x,y)-4b^2q(x,y)=0 in the triangular domain 0 < x < L-y < L,
with mixed boundary conditions. This solution is applied to the problem of
diffusion-limited coalescence, A + A <--> A, in the segment (-L/2,L/2), with
traps at the edges.
