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Re: Bug in Solve?

• To: mathgroup at smc.vnet.net
• Subject: [mg36846] Re: [mg36834] Bug in Solve?
• From: BobHanlon at aol.com
• Date: Sun, 29 Sep 2002 02:55:06 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

In a message dated 9/28/2002 10:57:40 AM, none at news.acns.nwu.edu writes:

>inside a program I need to solve this linear equation in terms of p1.
>However something odds happens. Sometimes the solution is computed and
>sometimes the result is empty [I mean no output...]. Is this a bug of the
>solve command or am I doing something wrong? The problem is robust to:
>changing name to the variables and other makeups..
>
>Thank you very much,
>
>David
>
>ps: Sorry for the stupid way in which I copied the command...
>
>
>
>Solve[(x^2*((-0.9*x^7*(p^2*(-1 - 5.8*x^6 - 14.010000000000002*x^12 -
>                              18.04*x^18 - 13.06*x^24 -
>                              5.040000000000001*x^30 - 0.81*x^36) +
>                        x*(7.777777777777779 - 9.074074074074076*x +
>                              30.333333333333336*x^6 -
>                              21.51851851851852*x^7 -
>                              16.333333333333336*x^8 +
>                              44.33333333333334*x^12 +
>                              3.188888888888883*x^13 -
>                              65.68333333333332*x^14 +
>                              28.777777777777786*x^18 +
>                              47.937037037037044*x^19 -
>                              100.10000000000002*x^20 + 7.*x^24 +
>                              45.6037037037037*x^25 -
>                              69.53333333333333*x^26 +
>                              13.299999999999999*x^31 -
>                              19.833333333333332*x^32 -
>                              1.0499999999999996*x^38) +
>                        p*(-6 + 8.296296296296296*x -
>                              28.799999999999997*x^6 +
>                              32.785185185185185*x^7 +
>                              9.333333333333336*x^8 -
>                              55.260000000000005*x^12 +
>                              49.04777777777776*x^13 +
>                              38.38333333333334*x^14 -
>                              52.980000000000004*x^18 +
>                              34.20518518518518*x^19 +
>                              60.20000000000001*x^20 -
>                              25.380000000000003*x^24 +
>                              11.736296296296294*x^25 +
>                              43.63333333333334*x^26 - 4.86*x^30 +
>                              2.8999999999999986*x^31 +
>                              13.533333333333333*x^32 + 0.81*x^37 +
>                              1.0499999999999996*x^38)))/(x + 1.9*x^7 +
>                      0.9*x^13)^2 - ((-1 + p - 7*x^6 + p*x^6 +
>                        6*x^7)*(1.2962962962962965 - 3.111111111111112*x^6
>+
>                        9.333333333333336*x^7 - 10.111111111111114*x^12
>+
>                        22.05*x^13 - 5.703703703703705*x^18 + 17.15*x^19
>+
>                        5.483333333333331*x^25 + 1.0499999999999996*x^31
>+
>                        p1*x^5*(7.000000000000002 - 7.000000000000002*x
>+
>                              14.000000000000004*x^6 -
>                              14.000000000000004*x^7 +
>                              7.000000000000002*x^12 -
>                              6.999999999999998*x^13) -
>                        1.166666666666667*p*x^4*x1 -
>                        3.500000000000001*p*x^10*x1 -
>                        1.0500000000000003*p*x^11*x1 -
>                        3.500000000000001*p*x^16*x1 -
>                        3.150000000000001*p*x^17*x1 -
>                        1.166666666666667*p*x^22*x1 -
>                        3.150000000000001*p*x^23*x1 -
>                        1.0500000000000003*p*x^29*x1))/((1 + 0.9*x^6)^2*(1
>+
>                          x^6)^2)))/(p^2*(1 + x^6)^3) == 0, p1]
>

You might find it more robust (and the results cleaner) if you Simplify the 
equation prior to using Solve.  Such as

Solve[eqn // Rationalize // Simplify, p1]

However, if you are assigning values to p or x prior to using Solve, there 
may not be a solution.  That is, for whenever the numerator of the expression 
for p1 would be zero, e.g., 

p = (-6 x^7 + 7 x^6 +1)/(x^6 + 1).


Bob Hanlon