How do I define this function with the WL? b[a_ , t_] := a'[t] Sin[a[t]];

Questions
1

Hi dears
does anyone know what is the problem when i define this function in mathematica?

b[a_ ,  t_] := a'[t] Sin[a[t]];

it gives :
SetDelayed::write: Tag Times in (sin’[a[t]] a’[t])[a_ ,t_] is Protected. >>

i want to define a function containing variables and their derivatives !
help me please :slight_smile:

2

Hi Amir,

most likely you first executed

b := a'[t] Sin[a[t]]

and then the correct definition

b[a_, t_] := a'[t] Sin[a[t]]

Do a clean start or at least execute

ClearAll[b]

then it will work.

Regards Henrik

3

It looks to me like you have previously defined b to be something else. Try clearing the variables a, b, t before defining the function.

The main clue that this is happening is they the error message uses sin (with lower case ‘s’) so it looks like b was defined as
(sin’[a[t]] a’[t]) and that is substituted into your function definition. Since this is right next to [a_, t_], Mathematica is trying to do an implicit multiplication.

I can enter your function without error on my system (MMa 10.1, OS X) in a fresh notebook.

4

Thank you Henrik
now it works correctly

5

Thank you George
now it works correctly

6

Dear Henrik
most recently i encountered with another problem in mathematica NDSolve
i had defined some equations “lagrangeo1” ,“lagrangei1”, and “lagrangei2” that are differential equations. but when started to solve:

lagrangesolve =
NDSolve[{lagrangei1 == 0, lagrangei2 == 0,
lagrangeo1 == 0,o1[0] == -36,i1[0] ==18,i2[0] == 18, o1’[0] == 0.5, i1’[0] == 0,i2’[0] == -0.75}, {o1, i1, i2}, {t, 0, 100}]

(o1,i1,i2 are my variables)
it said :

NDSolve::ndsdtc: The time constraint of 1.` seconds was exceeded trying to solve for derivatives, so the system will be treated as a system of differential-algebraic equations. You can use Method->{“EquationSimplification”->“Solve”} to have the system solved as ordinary differential equations. >>
NDSolve::njnum: The Jacobian is not a matrix of numbers at …
…
General::stop: Further output of NDSolve::njnum will be suppressed during this calculation. >>
NDSolve::icfail: Unable to find initial conditions that satisfy the residual function within specified tolerances. Try giving initial conditions for both values and derivatives of the functions. >>

will be glad if you could help me again. regards

7

Dear George
most recently i encountered with another problem in mathematica NDSolve
i had defined some equations “lagrangeo1” ,“lagrangei1”, and “lagrangei2” that are differential equations. but when started to solve:

lagrangesolve =
NDSolve[{lagrangei1 == 0, lagrangei2 == 0,
lagrangeo1 == 0,o1[0] == -36,i1[0] ==18,i2[0] == 18, o1’[0] == 0.5, i1’[0] == 0,i2’[0] == -0.75}, {o1, i1, i2}, {t, 0, 100}]

(o1,i1,i2 are my variables)
it said :

NDSolve::ndsdtc: The time constraint of 1.` seconds was exceeded trying to solve for derivatives, so the system will be treated as a system of differential-algebraic equations. You can use Method->{“EquationSimplification”->“Solve”} to have the system solved as ordinary differential equations. >>
NDSolve::njnum: The Jacobian is not a matrix of numbers at …
…
General::stop: Further output of NDSolve::njnum will be suppressed during this calculation. >>
NDSolve::icfail: Unable to find initial conditions that satisfy the residual function within specified tolerances. Try giving initial conditions for both values and derivatives of the functions. >>

will be glad if you could help me again. regards

8

Dears
most recently i encountered with another problem in mathematica NDSolve
i had defined some equations “lagrangeo1” ,“lagrangei1”, and “lagrangei2” that are differential equations. but when started to solve:

lagrangesolve =
NDSolve[{lagrangei1 == 0, lagrangei2 == 0,
lagrangeo1 == 0,o1[0] == -36,i1[0] ==18,i2[0] == 18, o1’[0] == 0.5, i1’[0] == 0,i2’[0] == -0.75}, {o1, i1, i2}, {t, 0, 100}]

(o1,i1,i2 are my variables)
it said :

NDSolve::ndsdtc: The time constraint of 1.` seconds was exceeded trying to solve for derivatives, so the system will be treated as a system of differential-algebraic equations. You can use Method->{“EquationSimplification”->“Solve”} to have the system solved as ordinary differential equations. >>
NDSolve::njnum: The Jacobian is not a matrix of numbers at …
…
General::stop: Further output of NDSolve::njnum will be suppressed during this calculation. >>
NDSolve::icfail: Unable to find initial conditions that satisfy the residual function within specified tolerances. Try giving initial conditions for both values and derivatives of the functions. >>

will be glad if anyone could help me. regards