Provided that the calculation sweeps in the direction
of increasing j, Uy, U;+ , and the advance time value
U;?,’ are already known, and eqn (2) can be conveniently
used to calculate UT+ ’ . Although it is a twolevel
method, it requires only one storage array, since
the same memory location can be used for U:+’ and
U; in the replacement statement [eqn (2)]. Since eqn
(2) can be solved for Uy+ I, the method is explicit.
To control the truncation error, the direction of
the calculation sweep is also reversed in the direction
decreasing j, giving a formula for V; = T: