## Re: [Reduce-algebra-developers] Question about df.

 Re: [Reduce-algebra-developers] Question about df. From: Rainer Schöpf - 2014-03-23 19:20:21 ```On Sun, 23 Mar 2014 at 23:50 -0000, abpetrov wrote: > > Hi, > > I try define a new operator f, and, respectively, a new derivation for it. > But there is an question. > > 1) > operator f\$ > operator x\$ > > df(f(x(i)),x(i)); > > This expression is not 0, it is right. > > 2) > operator f\$ > operator x\$ > > df(f(x(i)),x(k)); > > This expression is 0, it is not right. x(i) and x(k) are different, so there is no dependency between them. > I want have some unevaluated derivation, it is not 0. > > 3) I can define a rule, But it doesn't help. The result is 0. > operator f\$ > operator x\$ > operator Delta\$ > Deriv_by_ind := { df (x(~j), x(~j1)) => Delta(j-j1) }\$ > let Deriv_by_ind\$ > > df(f(x(i)),x(k)); > > How can I get unevaluated form df(f(x(i)),x(k)), which is not 0? Two possiblities: (a) define a rule for f: let { df(f(x(~j)),x(~k)) => df(f(x(j)),x(j)) * Delta(j-k) when j neq k}; (b) Use the dfpart package: load_package dfpart; generic_function f(u); operator Delta,x; Deriv_by_ind := { df (x(~j), x(~j1)) => Delta(j-j1) }; let Deriv_by_ind; Regards, Rainer ```