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Problem with Mathematica 9 or the equation is insoluble?

Posted 9 years ago

Hi, I have got this differential equation: y''=x*sin(y) I am trying to use Mathematica 9 to solve this equation, so I write the following expression: A = First[ y /. NDSolve[{y''[x] == x*Sin[y[x]], y[0] == 1, y'[0] == 1}, y, {x, 0, 19}]]

But Mathematica always answers with something like that:

"NDSolve::deqn: Equation or list of equations expected instead of True in the first argument {(y^[Prime][Prime])[x]==x Sin[y[x]],y[0]==1,True}. >>" And "ReplaceAll::reps: {NDSolve[{(y^[Prime][Prime])[x]==x Sin[y[x]],y[0]==1,True},y,{x,0,19}]} is neither a list of replacement rules nor a valid dispatch table, and so cannot be used for replacing. >>"

But a friend of mine has tried to input exactly what I wrote, and she got a normal result. She has Mathematica 10.

By answering a question, how many maximum and minimum has the function, the answer should be 8. And my question is: Is there any problem with Mathematica or can you see if I write something wrong?

Thanks to anyone who has tried solving this

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POSTED BY: Gregory Efimov
3 Replies

I think you may have had an uncleared variable.

In[1]:= sln = 
 NDSolve[{y''[x] == x*Sin[y[x]], y[0] == 1, y'[0] == 1}, y, {x, 0, 19}]

Out[1]= {{y -> InterpolatingFunction[{{0., 19.}}, <>]}}
POSTED BY: Frank Kampas
Posted 9 years ago

Since the error message includes

y^[Prime][Prime])[x]

that ^ makes me believe you are somehow putting your ' and ' into superscript notation and making Mathematica think you are taking y to the '' power.

If you start Mathematica fresh and just literally type only and nothing more than

A = First[ y /. NDSolve[{y''[x] == x Sin[y[x]], y[0] == 1, y[0] == 1}, y, {x, 0, 19}]]; Plot[A[x], {x, 0, 19}]

without any <ctrl><shift><alt> anything to make anything into a superscript and don't use any palettes to try to publish your notation, just type, or maybe even scrape off the screen and paste the text above, then I think the problem will go away

POSTED BY: Bill Simpson
Posted 9 years ago

Thank you for your recommendations! Your advices are very helpful

POSTED BY: Gregory Efimov
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