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2.14: Appendix I- Integrating Factors

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Suppose we have an inexact differential \mathchar26dW=Aidxi . Here I am adopting the ‘Einstein convention’ where we sum over repeated indices unless otherwise explicitly stated; Aidxi=iAidxi. An integrating factor  eL(x) is a function which, when divided into \mathchar26dF, yields an exact differential: dU=eL\mathchar26dW=Uxidxi . Clearly we must have 2Uxixj=xi(eLAj)=xj(eLAi) . Applying the Leibniz rule and then multiplying by eL yields AjxiAjLxi=AixjAiLxj . If there are K independent variables {x1,,xK}, then there are 12K(K1) independent equations of the above form – one for each distinct (i,j) pair. These equations can be written compactly as ΩijkLxk=Fij , where Ωijk=AjδikAiδjkiFij=AjxiAixj . Note that Fij is antisymmetric, and resembles a field strength tensor, and that Ωijk=Ωjik is antisymmetric in the first two indices (but is not totally antisymmetric in all three).

Can we solve these 12K(K1) coupled equations to find an integrating factor L? In general the answer is no. However, when K=2 we can always find an integrating factor. To see why, let’s call xx1 and yx2. Consider now the ODE dydx=Ax(x,y)Ay(x,y) . This equation can be integrated to yield a one-parameter set of integral curves, indexed by an initial condition. The equation for these curves may be written as Uc(x,y)=0, where c labels the curves. Then along each curve we have 0=dUcdx=Uxx+Ucydydxi=UcxAxAyUcy . Thus, UcxAy=UcyAxeLAxAy . This equation defines the integrating factor L: L=ln(1AxUcx)=ln(1AyUcy) . We now have that Ax=eLUcx,Ay=eLUcy , and hence eL\mathchar26dW=Ucxdx+Ucydy=dUc .


This page titled 2.14: Appendix I- Integrating Factors is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Daniel Arovas.

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