## reduce-algebra-developers — Discussion of development, administration and support for Reduce

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 [Reduce-algebra-developers] Commutation between sum and df. From: abpetrov - 2013-09-07 14:58:35 ```Hi, I tried to define an commutation rule for operation sum and operator df. It looks quite simple, but don't works. Why this rule don't work. Program with that rule is below, it looks simple, so without comments. off lower\$ operator H,k,x; H := sum( (1/2)*k(i)*x(i)**2, i, -infinity,infinity); r1 := {df(sum( ~~f, ~~i, ~~i1,~~i2),~~x(~~i)) => sum( df(f, x(i)) , i, i1,i2)}; (df(H,x(i)) where r1); end; Best regards, Petrov Alexander. ```

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