From Osher Doctorow
I typed this about 10 minutes ago, but it didn't post, so I'll just
outline in part what I typed.
The repeated (double) iterated integral of f(x,y) from x to infinity
and from y to infinity of f(x,y) is the only requirement to get R(x,y),
and the inner (dy) integral just requirements the substitution:
1) z = x + y + kxy (notice that x is constant with regard to dy)
2) dz = dy + kxdy = (1 + kx)dy
3) dy = (1/(1 + kx))dz (from (2))
4) I[u exp(au)]du = (1/a^2)(au - 1)exp(au) + constant
where I...du is the indefinite integral with respect to any variable u.
Then integrate the result with respect to x, and then get Dxy of this
result.
Osher Doctorow
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