Dear all, I have a quick question wrt the famous guy John Burkardt’s stochastic_rk.f90 code.

The main description is here,

https://people.math.sc.edu/Burkardt/f_src/stochastic_rk/stochastic_rk.html

The stochastic_rk.f90 code is below,

https://people.math.sc.edu/Burkardt/f_src/stochastic_rk/stochastic_rk.f90

Now, In the subroutine for RK4

rk4_ti_step ( x, t, h, q, fi, gi, seed, xstar )

in the lines,

t4 = t1 + a41 * h + a42 * h + a43 * h

x4 = x1 + a41 * k1 + a42 * k2

I wonder, should the expression for x4 be

x4 = x1 + a41 * k1 + a42 * k2 + a43 * k3

I mean, should there be the term + a43 * k3 for x4?

Does anyone know? Thank you very much!

Also, does anyone know how to contact John Burkardt? I can only find his webpage, but not his email address.