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Formal Calculation




However, this interpretation of the term .


EXAMPLES


A simple example

A somewhat exaggerated example would be to use the equation

:\sum_{n=0}^{\infty} q^n = rac{1}{1-q}

(which holds under certain conditions) to conclude that

:\sum_{n=0}^{\infty} 2^n = -1.

This is incorrect according to the usual definition of infinite sums of real numbers, since the related sequence does not converge. However, this result can inspire extending the definition of infinite sums, and the creation of new fields, such as the 2-adic Numbers , where the series in question converges and this statement is perfectly valid.


Formal power series

Formal Power Series is a concept that adopts some properties of convergent Power Series used in Real Analysis , and applies them to objects that are similar to power series in form, but have nothing to do with the notion of convergence.


Symbol manipulation

Suppose we want to solve the Differential Equation

: rac{dy}{dx} = y^2

Treating these symbols as ordinary algebraic ones, and without giving any justification regarding the validity of this step, we take reciprocals of both sides:

: rac{dx}{dy} = rac{1}{y^2}

Now we take a simple Antiderivative :

:x = rac{-1}{y} + C

:y = rac{1}{C-x}

Because this is a ''formal'' calculation, we can also allow ourselves to let C = \infty and obtain another solution:

:y = rac{1}{\infty - x} = rac{1}{\infty} = 0

If we have any doubts about our argument, we can always check the final solutions to confirm that they solve the equation.


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REFERENCES