115 views

Skip to first unread message

May 9, 2014, 2:08:06 AM5/9/14

to

Hi, could somebody try this in Mathematica 9 to see if the bug is fixed?

sol = DSolve[{u'[t] == 2*Sqrt[u[t]], u[0] == 0}, u[t], t]; Print[sol];

Thanks.

sol = DSolve[{u'[t] == 2*Sqrt[u[t]], u[0] == 0}, u[t], t]; Print[sol];

Thanks.

May 14, 2014, 5:25:37 AM5/14/14

to

Am 09.05.2014 08:08, schrieb carlos.felippa%colora...@gtempaccount.com:

> Hi, could somebody try this in Mathematica 9 to see if the bug is fixed?

>

> sol = DSolve[{u'[t] == 2*Sqrt[u[t]], u[0] == 0}, u[t], t]; Print[sol];

In all versions until now, Mathematica does not check the Lipshitz
> Hi, could somebody try this in Mathematica 9 to see if the bug is fixed?

>

> sol = DSolve[{u'[t] == 2*Sqrt[u[t]], u[0] == 0}, u[t], t]; Print[sol];

condition |f(u(t))-f(u(0))| < C |u(t)-u(0)| that guaranties uniqueness.

The results for nonlinear ordinary differential equations are just those

you find in the usual lists like Kamke oder EquationWorld.

There exist no general algebraic nonlinear solving methods for nonlinear

ODE's except linear substitutions, separation of variables and lookup

tables.

Generally as a student of ODE one learns to look for points of

discontinuities, here u=0, which generally allow branching:

f(t):=0 /;t<=t0

f(t):=(t-t0)^2/; t>t0>=0

is the solution family on (-oo,oo).

Dsolve has no entry for a domain construct like NDSolve

NDSolve[{u'[t] == 2*Sqrt[u[t]], u[0] == 0}, u[t],{t,0,10}]

that will give you the missing constant solution and. But

In[20]:= v[t_] =

u[t] /. NDSolve[{u'[t] == 2 Sqrt[u[t]], u[1] == 10^-12},

u[t], {t, 0, 7}][[1]]

There occurs an error in the internal procedure at the critical point 1:

>>During evaluation of In[20]:= NDSolve::mxst: Maximum number of 10000

>>steps reached at the point t == 0.9998803430478553`.

So even with CASystems, it is still the fact that differentiation is an

algebraic functor, numerical integration is a trivialtity for smooth

functions, algebraic integration is a mystery and a historical source of

many branches of mathematics.

The solution of nonlinear differential equations remains an ingenious

kind of art, Ricatti, Clairaut, d'Alembert are some of the protagonists.

--

Roland Franzius

Reply all

Reply to author

Forward

0 new messages

Search

Clear search

Close search

Google apps

Main menu