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calculus and analysis - How to do algebra on unevaluated integrals?


I am working with functions calculated from a set of general basis functions.



f = a[x] + c1*b[x] + c2*c[x];
g = Expand[f*f];
h = Integrate[g, {x, -Infinity, Infinity}];

This much works fine, and I get a nice expression with products of combinations of a[x], b[x], and c[x] in the integrand. But now I need to calculate the values of c1 and c2 that optimize h and the optimal value of h. This fails:


cs = {c1, c2};
s = Solve[{D[h, #] & /@ cs == 0}, cs]
(* Solve::nsmet: This system cannot be solved with the methods available to Solve. >> *)

Is there any hack to make this work?



EVEN better would be if I could get the answer in tidy bracket notation, where for example denotes the definite integral of a[x]*b[x].



Answer



Similar idea to belisarius, except in V10 we can inactivate Integrate to keep it from evaluating or even trying to evaluate:


h = Inactive[Integrate][g, {x, -Infinity, Infinity}]

It is not necessary in this example, as belisarius' answer shows, but one of its intended uses is to do algebra/calculus on integrals and derivatives. Inactive can be removed easily with


Activate[h]

The function linearExpand expands its argument according to linearity properties. Factors/terms that do not depend on x are treated as constants (see update below for a more general approach).


Clear[linearExpand];

linearExpand[e_] := e //. {int : Inactive[Integrate][_Plus, _] :> Distribute[int],
Inactive[Integrate][integrand_Times, dom : {x_, _, _}] :>
With[{dependencies = Internal`DependsOnQ[#, x] & /@ List @@ integrand},
Pick[integrand, dependencies, False] *
Inactive[Integrate][Pick[integrand, dependencies, True], dom]
]};

OP's sample problem:


Solve[D[h, #] == 0 & /@ cs // linearExpand, cs]


Mathematica graphics


D[h, #] == 0 & /@ cs // linearExpand

Mathematica graphics




For what it's worth...


...here's a general linearity expander. Considers factors that do not depend on x, which may be a list of symbols, as constants.


linearExpand[e_, x_, head_] := 
e //. {op : head[arg_Plus, __] :> Distribute[op],
head[arg1_Times, rest__] :>

With[{dependencies = Internal`DependsOnQ[#, x] & /@ List @@ arg1},
Pick[arg1, dependencies, False] head[
Pick[arg1, dependencies, True], rest]
]};

Examples:


linearExpand[D[h, #] == 0 & /@ cs, x, Inactive[Integrate]]
(* same as above *)

linearExpand[foo[(a[x] + c b[y]) (2 a[x] - c b[y]) // Expand, randomarg], x, foo]

(* -c^2 b[y]^2 foo[1, randomarg] +
c b[y] foo[a[x], randomarg] +
2 foo[a[x]^2, randomarg] *)

linearExpand[foo[(a[x] + c b[y]) (2 a[x] - c b[y]) // Expand, randomarg], {x, y}, foo]
(* 2 foo[a[x]^2, randomarg] +
c foo[a[x] b[y], randomarg] -
c^2 foo[b[y]^2, randomarg] *)

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