Search found 11 matches

by Bobobo
Tue Dec 08, 2009 9:44 pm UTC
Forum: Mathematics
Topic: Matrices and how they transform graphs
Replies: 28
Views: 2875

Re: Matrices and how they transform graphs

you're right, matrix multiplication is in general not commutative (order matters). reflecting then rotating gives something different to rotating then reflecting. a minor point, AS=SA for all A iff S is a scalar multiple of the identity matrix. You might like to try proving this yourself, it's not ...
by Bobobo
Tue Dec 08, 2009 9:40 pm UTC
Forum: Mathematics
Topic: Matrices and how they transform graphs
Replies: 28
Views: 2875

Re: Matrices and how they transform graphs

Scalar multiple as in a*I where a is a scalar and I is the identity matrix. so, for example a matrix with all 5's on the diagonal and nothing else
by Bobobo
Tue Dec 08, 2009 8:56 am UTC
Forum: Mathematics
Topic: Matrices and how they transform graphs
Replies: 28
Views: 2875

Re: Matrices and how they transform graphs

you're right, matrix multiplication is in general not commutative (order matters). reflecting then rotating gives something different to rotating then reflecting. a minor point, AS=SA for all A if and only if S is a scalar multiple of the identity matrix. You might like to try proving this yourself,...
by Bobobo
Thu Nov 12, 2009 5:47 am UTC
Forum: Mathematics
Topic: Simple Factoring...
Replies: 7
Views: 1084

Re: Simple Factoring...

a short way to do the second:
x^4 + 8x^2 + 144 = (x^2+12)^2 - 16x^2 = (x^2+12-4x)(x^2+12+4x)

the last step is difference of 2 squares.
by Bobobo
Wed Nov 11, 2009 6:43 am UTC
Forum: Mathematics
Topic: Ten Problems for Geometrical Intuition
Replies: 58
Views: 5827

Re: Ten Problems for Geometrical Intuition

re q4 see pic: http://img23.imageshack.us/img23/9899/circlel.jpg Note that sliding the stick around is equivalent to moving the point on the circumference around. (remember that high school theorem, any triangle in a circle with diameter as one side and the other point on the circumference is a ...
by Bobobo
Thu Oct 29, 2009 11:21 am UTC
Forum: Mathematics
Topic: Apparently easy complex number problem
Replies: 20
Views: 2136

Re: Apparently easy complex number problem

just let z / (z - i) = r where r is real, then solve for z.
by Bobobo
Tue Oct 13, 2009 9:34 am UTC
Forum: Mathematics
Topic: Becoming a Mathematician?
Replies: 17
Views: 4676

Re: Becoming a Mathematician?

"If nothing else, universities around the world (but particularly under the US system) need huge numbers of people to teach maths to undergrads, and not just those undergrads specialising in maths either."

isn't that what phds students and postdocs are for though?
by Bobobo
Wed Oct 07, 2009 12:59 pm UTC
Forum: Mathematics
Topic: Expression that alternates between 0 and 1?
Replies: 13
Views: 5081

Re: Expression that alternates between 0 and 1?

i'm curious, what was the question?
by Bobobo
Wed Aug 05, 2009 12:55 pm UTC
Forum: Mathematics
Topic: Book Club: Mathematics: Its Contents... discussion
Replies: 50
Views: 5349

Re: Book Club: Mathematics: It's Contents... discussion

you guys might find mathim.com useful. real time chat + latex, no rego required. bit laggy though.
by Bobobo
Sat May 02, 2009 5:30 am UTC
Forum: General
Topic: Who the dickens are you?
Replies: 10922
Views: 2381305

Re: INTRO THREAD THE THIRD

hi, my name is Bobo, 17/male/melbourne ive been into maths and programming since about 15, other recent interests include unicyling and playing recorder i'm in my first year of science/mechanical engineering at monash uni, but i'm starting to have second thoughts about the engineering part. proving ...
by Bobobo
Sun Sep 14, 2008 10:47 am UTC
Forum: Coding
Topic: Project Euler
Replies: 411
Views: 158714

Re: Project Euler

why is my sieve so slow? the one above me takes 0.0940001010895 seconds to run, for n = 100,000 whereas mine takes 85.8440001011: import time def prsieve(N): 'Returns all primes <= N in list prim' prim = [] prim = range(2,N+1) n = 0 p = 0 while p <= N**.5: p = prim[n] for x in prim: if x%p == 0 and ...

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