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Iterated Integrals

    In this topic we evaluate many iterated integrals working through examples in a step-by-step manner. As with single integrals, it is often not practical to evaluate a double integral even over a simple rectangular region by using the definition. Instead, we compute double integrals by a process called iterated integration, which is like partial differentation in reverse. Suppose iterated integrals _gr_1.gif] is continuous over the rectangle iterated integrals _gr_2.gif] Then we write   iterated integrals _gr_3.gif] to denote the integral obtained by integrating iterated integrals _gr_4.gif] with respect to iterated integrals _gr_5.gif] over the interval iterated integrals _gr_6.gif] with iterated integrals _gr_7.gif] held constant. The integral obtained by this partial integration is a function of iterated integrals _gr_8.gif] alone,   iterated integrals _gr_9.gif] which we integrate over the interval iterated integrals _gr_10.gif] to obtain the iterated integral

iterated integrals _gr_11.gif]

Similarly, if we integrate iterated integrals _gr_12.gif] first with respect to iterated integrals _gr_13.gif] over iterated integrals _gr_14.gif] holding iterated integrals _gr_15.gif] constant, and then with respect to iterated integrals _gr_16.gif] over iterated integrals _gr_17.gif] we obtain the iterated integral,  

iterated integrals _gr_18.gif]

Fubini's Theorem says that these double integrals have the same value, provided iterated integrals _gr_19.gif] is continuous over iterated integrals _gr_20.gif] (in fact, Fubini's theorem say more).

Proposition (Fubini's Theorem) If iterated integrals _gr_21.gif] is continuous over the rectangle iterated integrals _gr_22.gif] iterated integrals _gr_23.gif] iterated integrals _gr_24.gif] then the double integral iterated integrals _gr_25.gif] may be evaluated by either iterated integral; that is,  

iterated integrals _gr_26.gif]

Example (Iterated Integrals) Use iterated integration to compute the double integral   iterated integrals _gr_27.gif] where iterated integrals _gr_28.gif] iterated integrals _gr_29.gif] iterated integrals _gr_30.gif]

    Solution. We compute  

iterated integrals _gr_31.gif]
iterated integrals _gr_32.gif]

Example (Iterated Integrals) Use iterated integration to compute the double integral iterated integrals _gr_33.gif] where iterated integrals _gr_34.gif] iterated integrals _gr_35.gif] iterated integrals _gr_36.gif]

    Solution. We compute  

iterated integrals _gr_37.gif]

iterated integrals _gr_38.gif]

Example (Iterated Integrals) Use iterated integration to compute the double integral iterated integrals _gr_39.gif] where iterated integrals _gr_40.gif] iterated integrals _gr_41.gif] iterated integrals _gr_42.gif]

    Solution. We compute

iterated integrals _gr_43.gif]
iterated integrals _gr_44.gif]

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Iterated Integrals
Published by Library of Math -- Online math organized by subject into topics.
Written by Smith, David A.
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