An equation is said to be a solution in closed form if it solves a given problem in terms of functions and mathematical operations chosen from a limited and generally accepted set. The qualification of a closed form is somewhat arbitrary, since it depends greatly on the predefined set of operations and functions. For example, a series (the sum of all terms of an infinite sequence) is not usually considered closed form; unless a summation operator of infinite terms in a series is included in the set of operators.
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