DSolve bug in 7.0.0
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From: Zachar István on 4 Jun 2010 08:02 Dear All, the following call gives different results under different Mathematica versions. DSolve[{x'[t] == k x[t]^p, x[0] == c}, x[t], t] On one Mathematica, it produces a solution, but not in the other. There are no autoloaded packages on either of the machines. Under Mathematica 7.0.1.0 (Lenovo ThinkPad X200 Intel Core 2 Duo CPU 2.27 GHz 2.27GHz with Win 7), and Mathematica 6 (on a Sony Vaio with Vista) the result is: -----------------[code start]----------------- In[1]:= DSolve[{x'[t] == k x[t]^p, x[0] == c}, x[t], t] Solve::ifun: Inverse functions are being used by Solve, so some solutions may not be found; use Reduce for complete solution information. >> Solve::ifun: Inverse functions are being used by Solve, so some solutions may not be found; use Reduce for complete solution information. >> Out[1]= {{x[t] -> ((-1 + p) (c^(1 - p)/(-1 + p) - k t))^(1/(1 - p))}} -----------------[code end]----------------- Under Mathematicah 7.0.0 (HP EliteBook 8440p Intel Core i5 CPU M 520 @ 2.4 Ghz 2.4Ghz, with Win 7): -----------------[code start]----------------- In[1]:= DSolve[{x'[t] == k x[t]^p, x[0] == c}, x[t], t] During evaluation of In[2]:= Solve::ifun: Inverse functions are being used by Solve, so some solutions may not be found; use Reduce for complete solution information. >> During evaluation of In[2]:= Solve::ifun: Inverse functions are being used by Solve, so some solutions may not be found; use Reduce for complete solution information. >> During evaluation of In[2]:= Solve::ifun: Inverse functions are being used by Solve, so some solutions may not be found; use Reduce for complete solution information. >> During evaluation of In[2]:= General::stop: Further output of Solve::ifun will be suppressed during this calculation. >> During evaluation of In[2]:= DSolve::bvnul: For some branches of the general solution, the given boundary conditions lead to an empty solution. >> Out[1]= {} -----------------[code end]----------------- I've just tried it under Mathematica 7.0.1.0 in the very same HP EliteBook machine, and it gives the correct solution (the first one), thus it is a version-specific bug. What happened after 6.0 that causes 7.0.0 to fail? And was it intentionally fixed in 7.0.1.0? I could not find any information about this on the internet... Is this bug (or at least inconsistency) known to others? Has anyone discovered it before? The strange thing is that the documentation for DSolve AND Solve (as the error messages refer to Solve which, I guess, is used internally) state that the DSolve and Solve functions haven't been changed since version 5. Istvan |