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Three questions1) Let's say that N ##q_i## and ##p_i## are transformed into ##Q_k## and ##P_k##, so that:
##q_i = q_i(Q_1,Q_2,. ... , P_1,P_2, ... ) ## and ##p_i=p_i((Q_1,Q_2,. ... , P_1,P_2, ... )##
We have proved that these transformations are canonical only and only if ##\forall i##
##\{Q_k(q_i,p_i),H(q_i,p_i,t)\}_{p,q} = \{Q_k, H(Q_k,P_k,t)\}_{Q,P}## (1)
Until now I understand why this is the case. Next step in the reasoning was saying :
And thus, for any functions ##f(q_i,p_i)## and ##g(q_i,p_i)## must hold ##\{f,g\}_{q,p}= \{f,g\}_{Q,P}## (2)
How to make this final step from (1) to (2) ?
2) We also use the term ''transformed Hamiltonian'' ##K(Q_i,P_i,t)## for which the Hamilton equations hold for ##Q_i## and ##P_i##
I'm pretty sure that it's basically the old hamiltonian ##H(q_i,p_i,t)## with all the ##q_i## and ##p_i## expressed in function of the new variables but the new letter ##K## makes me doubt it a little. If it's the same thing why use a different letter? So just asking to be sure here.
3) Why all of a sudden restrictions on variables we can transform into? In Lagrangian mechanics one can transform in any generalized coordinates ( as long as they are time independent if you want to have nice properties ) but basically no real restrictions.
Suddenly in Hamiltionian mechanics you need to check what I mentioned above. I find it weird since Lagrangian and Hamiltonian formalisms should be equivalent in power. Is there an intuitive reason why one has no restrictions on ( basic ) transformations and the other does?
##q_i = q_i(Q_1,Q_2,. ... , P_1,P_2, ... ) ## and ##p_i=p_i((Q_1,Q_2,. ... , P_1,P_2, ... )##
We have proved that these transformations are canonical only and only if ##\forall i##
##\{Q_k(q_i,p_i),H(q_i,p_i,t)\}_{p,q} = \{Q_k, H(Q_k,P_k,t)\}_{Q,P}## (1)
Until now I understand why this is the case. Next step in the reasoning was saying :
And thus, for any functions ##f(q_i,p_i)## and ##g(q_i,p_i)## must hold ##\{f,g\}_{q,p}= \{f,g\}_{Q,P}## (2)
How to make this final step from (1) to (2) ?
2) We also use the term ''transformed Hamiltonian'' ##K(Q_i,P_i,t)## for which the Hamilton equations hold for ##Q_i## and ##P_i##
I'm pretty sure that it's basically the old hamiltonian ##H(q_i,p_i,t)## with all the ##q_i## and ##p_i## expressed in function of the new variables but the new letter ##K## makes me doubt it a little. If it's the same thing why use a different letter? So just asking to be sure here.
3) Why all of a sudden restrictions on variables we can transform into? In Lagrangian mechanics one can transform in any generalized coordinates ( as long as they are time independent if you want to have nice properties ) but basically no real restrictions.
Suddenly in Hamiltionian mechanics you need to check what I mentioned above. I find it weird since Lagrangian and Hamiltonian formalisms should be equivalent in power. Is there an intuitive reason why one has no restrictions on ( basic ) transformations and the other does?
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