# Recent content by xsw001

1. ### Solving Nonlinear System using Matlab

Homework Statement Find the stable/unstable manifold for the nonlinear system dx/dt=y^2-(x+1)^2; dy/dt=-x Homework Equations The Attempt at a Solution I'm trying to solve the below nonlinear system using Matlab, but got the following warning message. Any idea...
2. ### How to construct nonlinear ODE systems with given condition?

I have a general question about how to construct nonlinear ODE systems with given condition such as # of critical points with certain characteristics of the phase portrait of each critical point. I have no problem solving any type of nonlinear ODE system. But to do the reverse order, I have...
3. ### H is a subgroup of its Centralizer iff H is Abelian

Yeah, that's true, as simple as it is. Doesn't even need any algebraic proof, just simply explain it out then.
4. ### H is a subgroup of its Centralizer iff H is Abelian

H < CG(H) <=> H is Abelian CG(H) is the centralizer of H in G. Being a centralizer of H in G, just saying every element of H commute with every element of G. It does NOT say anything about the relationship between elements inside the H. We need to use the fact that H is a subgroup of G and...
5. ### H is a subgroup of its Centralizer iff H is Abelian

Homework Statement G is a group, and H is a subgroup of G. (1) Show H is a subgroup of its Normalizer. Give an example to show that this is NOT necessarily true if H is NOT a subgroup. (2) Show H is a subgroup of its Centralizer iff H is Abelian Homework Equations normalizer NG(H) = {g in...
6. ### How to find the order of a matrix?

For example the Heisenberg group over Field. H(F) is a 3x3 upper right triangle where the entries on the main diagonals are all 1's. So by definition that I need to use this matrix and raise to the power where it becomes the identity matrix, then the number of that power would the...
7. ### How to count the total # of non-invertible 2x2 matrices

Can anyone explain to me how to count the total # of non-invertible 2x2 matrices? I have the answer from the book, which is r^3+r^2-r provided r is a prime. But it doesn't explain how to get there, and I couldn't figure it out. I haven't been practicing linear algebra for quite a long...
8. ### Prove # of m-cycles in Sn (symmetric group)

Cool! Thanks tiny-tim!
9. ### Prove # of m-cycles in Sn (symmetric group)

Okay, for any given Sn, there are n elements in Sn {1, 2, ... m,..., n}, so we have n choices for the 1st element, then n-1 choices for the 2nd element, so on and so forth, and we have n-k+1 choices for mth element, etc. So there are total of n(n-1)(n-2)...(n-m+1)choices to form a m-cycle, to...
10. ### Prove # of m-cycles in Sn (symmetric group)

Homework Statement Prove that if n>=m then the # of m-cycles in Sn is given by [n(n-1)(n-2)...(n-m+1)]/m Homework Equations The order of Sn is n!. We're counting the # of ways of forming an m-cycle, then divide by the # of a particular m-cycle. The Attempt at a Solution This problem...
11. ### Find the order of the cyclic subgroup of D2n generated by r

Homework Statement Find the order of the cyclic subgroup of D2n generated by r. Homework Equations The order of an element r is the smallest positive integer n such that r^n = 1. Here is the representation of Dihedral group D2n = <r, s|r^n=s^2=1, rs=s^-1> The elements that are in D2n...
12. ### Prove t = x*y => t*x = x*t^(-1)

Yes! Got it! Since t^-1=(y^-1)*(x^-1) and x^2=1 => x=x^-1, y^2=1 => y=y^-1, so t^-1=yx Then tx=xyx, and x(t^-1)=xyx, indeed they are equal! Thanks for the tips Delta! Now it seems so simple!
13. ### Prove t = x*y => t*x = x*t^(-1)

Let x, y be elements of order 2 in any group G. Prove that if t = xy, then t*x = x*t^(-1) Here is what I got so far. Proof: Since |x| = 2 => x^2 = 1; |y| = 2 => y^2 = 1, then (x^2)(y^2) = 1 => (xy)^2 = 1 Suppose t = xy, then t^2 = (xy)^2 = 1 WTS (want to show) t*x = x*t^(-1) This group looks...
14. ### Prove x^2=1 if and only if the order of x is 1 or 2

So assume x^2=1, By definition of order of element if 2 is the smallest positive integer then order of x is 2. Otherwise 2 is not the smallest, then 1 would be the only smallest positive integer, then the order of element x would be 1. Does that look like a valid proof?
15. ### Prove x^2=1 if and only if the order of x is 1 or 2

G is a group. Let x be an element of G. Prove x^2=1 if and only if the order of x is 1 or 2. How do I approach this problem? I know since G is a group, all the elements in there have the following four properties: 1) Closure: a, b in G => a*b in G 2) Associative: (a*b)*c=a*(b*c) 3)...