Working with Operators
You can think of an expression like

as being formed by applying an operator
f to the expression
x. You can think of an expression like

as the result of
composing the operators
f and
g, and applying the result to
x.
Some functional operations.
This represents the composition of the functions
f,
g, and

.
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You can manipulate compositions of functions symbolically.
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The composition is evaluated explicitly when you supply a specific argument.
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You can get the sum of two expressions in
Mathematica just by typing

. Sometimes it is also worthwhile to consider performing operations like addition on
operators.
You can think of this as containing a sum of two operators
f and
g.
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Using
Through, you can convert the expression to a more explicit form.
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This corresponds to the mathematical operator

.
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Mathematica does not automatically apply the separate pieces of the operator to an expression.
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You can use
Through to apply the operator.
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Operations for working with operators.
This has a complicated expression as a head.
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Functions like
Expand do not automatically go inside heads of expressions.
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The replacement operator

does go inside heads of expressions.
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You can use
Operate to apply a function specifically to the head of an expression.
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