Finding terms with derivatives

Questions
1

Hi
I have a list of equations
eqns={lhs1==rhs1,lhs2==rhs2,…}
I want to find those equations containing terms using the derivative operator ('). Could this be done??

Thanks in advance.
J. Rico

2

(* Mathematica 7 )
( Clear memory )
Clear["Global`"];
(* Define a list of equations. *)
eqns = {x == 2y, x’[t] == 2t, x’‘[t] == 4t +1, y’[s] == 2z, y == t, y[m] == 2m, g[t] == x’[t]}

(* To find all equations with any derivative. *)
anyD = Position[ eqns, Derivative] (* Find positions. *);
pos  = Take[#, 1][[1]]& /@ anyD    (* Isolate first "address" element. *);
disp = Take[ eqns, {#}]& /@ pos    (* Print out all eqns with any derivative. *)

(* To find equations with a specified derivative. *)
specificD = Position[ eqns,#]& /@ { _'''}      (* Find positions. *);
pos       = Take[#, 1][[1]]& /@ specificD[[1]] (* Isolate the "address" element. *);
disp      = Take[ eqns, {#}]& /@ pos           (* Print out the equations. *)
3

(* Mathematica 7, Compact code. )
( Substitute without quotes, “Derivative” for all derivatives,
" _’ " for first derivative, " _‘’ " for second derivative etc.*)

Take[ eqns, {#}]& /@ (Flatten[ Take[#, 1]& /@ Position[ eqns, #]])& /@ 
{_''''(* <--- User substitutions here.*)}
4

Isaac
Thanks for the several alternatives. They work very well!
Jesus

5

Isaac,
In your code for finding specific derivatives

specificD = Position[ eqns,#]& /@ { _'''}      (* Find positions. *);

the result is a list of positions. How can I use them to extract the actual derivative e.g. x’[t]. The positions are lists and when I give them as arguments of [[…]] the result is not the expected.
This may be a silly question, sorry!

6

I have solved the problem with extract… sorry for the silly question.

Thanks!

Jesus