Re: Taking LCM in a polynomial expression

*To*: mathgroup at smc.vnet.net*Subject*: [mg95354] Re: Taking LCM in a polynomial expression*From*: dh <dh at metrohm.com>*Date*: Fri, 16 Jan 2009 06:10:14 -0500 (EST)*References*: <gkn5pa$iob$1@smc.vnet.net>

Hi Srikanth, the magic word to get a common denominator is: Together. E.g.: Together[num1/den1 + num2/den1] The second question is tougher. Mathematica orders increasingly by default. You therefore have to prevent this by e.g. HoldForm. Try: a2 x^2 + a1 x + a0 and HoldForm[a2 x^2 + a1 x + a0] Next question is how to get the polynomial inside HoldForm. Assume p=a0 + a1 x + a2 x^2 a polynomial in default ordering. We may get its coefficients with CoefficientList. The function Reverse reverses the coefficients list cof=Reverse@CoefficientList[p, x] the powers of x in our ordering: pow=Table[x^i, {i, 2, 0, -1}] The polynomial term in a list: pol= cof pow Now we insert pol into HoldForm by substitution and change List into Plus: HoldForm[dummy]/.dummy->pol /. List->Plus All together: p=a0 + a1 x + a2 x^2; cof=Reverse@CoefficientList[p, x]; pow=Table[x^i, {i, 2, 0, -1}]; pol= cof pow; HoldForm[dummy]/.dummy->pol /. List->Plus this gives: a2 x^2+a1 x+a0 hope this helps, Daniel Srikanth wrote: > Hi > I have a symbolic matrix that I invert. When I take each individual > entry of the resulting matrix, I get an expression of the form: > num1/den1 + num2/den1 + .... + num_n/den1 > where den1 is the determinant of the matrix. I'd like to get an > expression of the form: > Num/Den. > > I tried using Simplify (as well as multiplying the matrix by its > determinant), but it doesn't seem to help. Any suggestions? > > On a related note, the final Num is a polynomial in two variables - > say x,y. I'd like to arrange it in decreasing powers of x - like f1(y) > x^n + f2(y) x^n-1... + constant. What function should use? Is there > any way to both in a single step? > > Thanks a lot > Srikanth >