# Homework Help: Calculus by variations Euler's eq

1. Sep 15, 2011

### Liquidxlax

1. The problem statement, all variables and given/known data

This is for Classical mechanics 2

I'm not sure how to put partials in itex and the N with the dot beside it was supposed to be the derivative of N with respect to time

Suppose that you have N0 shares of stock and you want to make as money as you can by selling all of them in a single day. If N0 is large, and you sell your shares in small batches, the money you make can be written apporximately as an integral

$\int dt N^{.}P(N,N^{.};t)$ t1 to t2

where t1 and t2 are the opening and closing times for the stock exchange, N(t) is a smooth function that is approximately the number of shares you have sold at time t (which satisfies N(t1) = 0 and N(t2) = N0), N^{.} is the rate at which you sell the stock (in shares per hour, say) and P(N, N^{.};t) is the price per share as a function of time. The interesting thing is that the price depends on how you sell the shares. For example, if you selll them too fast, the price will drop. That is why P depends on N and N^{.}.

a) Suppose that P(N,N^{.};t) = P0 - Bt -CN^{.} for P0, B, C all positive (This is a "bear market" because of the -Bt term as the stock price is going down with time). Find N(t) and N^{.} that allow you to make the most money.

b) Discuss briefly what happens if B is too large.

2. Relevant equations

$\frac{dP}{dN} - \frac{d}{dt}(\frac{dP}{dN^{.}} = 0$

3. The attempt at a solution

I've really got nothing for this because i'm unsure of how to start minus checking the relevant equation

$\frac{dP}{dN} = 0 = \frac{d}{dt}\frac{dP}{dN^{.}}$

so

$\frac{dP}{dN^{.}} = -C$

and i think there has to be an auxillary equation to do with the fact that N(t1) = 0 and N(t2) = N0
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

Last edited: Sep 15, 2011
2. Sep 15, 2011

### Liquidxlax

can anyone give me an idea