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integration

# integration - INTEGRATION LECTURE NOTES by E B Davies 1...

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INTEGRATION LECTURE NOTES by E B Davies 1. Introduction If f is a real-valued function on a bounded interval [ a, b ] the integral b a f ( x ) d x is intended to be a measure of the area under the graph of the function. If the function takes both positive and negative values then one defines the integral to be the difference of the areas of A and B where A = { ( x, y ) : a x b and 0 < y < f ( x ) } B = { ( x, y ) : a x b and 0 > y > f ( x ) } . The above idea of integral transfers the problem of giving a precise definition of integral to that of giving a precise definition of area, which turns out to be no easier. We shall not attempt to define integration from first principles in these notes, but concentrate on the properties that a successful integration procedure should have, and then develop the theory from there. The first issue is that one cannot hope to integrate every conceivable function, ob- taining a well-defined real number as its integral. In some cases the integral is infinite, in others both sets A and B have infinite areas, and there is no sensible meaning to ∞ - ∞ , while in yet others the function may be so irregular that it is not clear how to start to define its integral. In these notes we only consider the integrals of piecewise continuous functions on a bounded interval [ a, b ]. Piecewise continuous functions f : [ a, b ] R are functions for which there exist numbers a = a 0 < a 1 < · · · < a n = b such that f is continuous on each interval ( a k - 1 , a k ) and the limits lim x a k - 0 f ( x ) , lim x a k +0 f ( x ) exist for all relevant k . In other words, this means that f has a jump discontinuity of size f ( a k +0) - f ( a k - 0) at each point a k , but is otherwise continuous. The space PC of such functions contains the space S of step functions and the space C of continuous functions on [ a, b ] (recall that a step function f is a piecewise continuous functions in the sense of our definition, which takes constant values on each interval ( a k - 1 , a k )). It is evident from the definition that if f and g are piecewise continuous functions on [ a, b ] and α, β R then the combination ( αf + βg )( x ) = αf ( x ) + βg ( x ) is also piecewise continuous. The set of jump points of αf + βg is just the union of the jump points of f and of g , or possibly a smaller set if the new function has a jump of size zero between two consecutive intervals. We summarize this by saying that PC is a vector space of functions. It is not finite-dimensional because it contains the polynomials of all orders.

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integration - INTEGRATION LECTURE NOTES by E B Davies 1...

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