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#2 Help Me ! » Minimum of the function » 2012-03-26 22:30:04

onako
Replies: 3

Given the following function


I would like to find a value c for which the above function is minimal.
Setting the above derivative to zero (with respect to c) should work, but I'm not sure how to handle
squares in a summation.

#3 Re: Help Me ! » Trying to reduce the value of a quadratic function » 2012-03-03 00:31:13

Given that all the eigenvalues of I-B, where I is the identity matrix, satisfy


what would be the range for the eigenvalues of B?

#4 Re: Help Me ! » Trying to reduce the value of a quadratic function » 2012-03-01 03:06:30

OK, I reduced the problem to the following: Suppose a square matrix A is given, with D being a diagonal matrix containing only its diagonal entries
which are positive.

It is known that the eigenvalues of


are bounded by 1 in magnitude (I denotes the identity matrix), ie.
,
where
is the i-th eigenvalue of
. How could the following be proved:
eigenvalues of

are all non-positive? Note that
corresponds to
with square rooted values.

#5 Re: Help Me ! » Trying to reduce the value of a quadratic function » 2012-02-28 04:29:42

The function is f(x), resulting in a value, it is clear that
x_0, x_1, and b are column vectors, and their transposes are row vectors.
(vectors are oriented to allow the operations)

#6 Re: Help Me ! » Trying to reduce the value of a quadratic function » 2012-02-28 02:23:37

Sorry, I thought the notation would suffice.
x and b are vectors;
A, R, D are matrices;

as for the second question: suppose a square matrix A is given.
Now split A = D + R, such that D is a square diagonal matrix with diagonal entries of A
(hence, the matrix R entries would be the off-diagonal entries of A)

#7 Help Me ! » Trying to reduce the value of a quadratic function » 2012-02-28 01:09:52

onako
Replies: 7

I'm trying to resolve the following:
given a function


and the update

where A = D+R, with the being the diagonal matrix with diagonal entries of A (and R containing the off-diagonal entries of A), how could I show

#8 Re: Help Me ! » Proving the ineqautity involving summation » 2012-02-27 04:06:05

I apologize for not making it precise.
All the elements


and

Perhaps this changes the conclusion.

#9 Help Me ! » Proving the ineqautity involving summation » 2012-02-27 03:39:17

onako
Replies: 3

Suppose the following relation holds


Can it be shown that the equivalence (if and only if) holds for

#10 Help Me ! » One optimization to facilitate another » 2012-01-31 03:27:23

onako
Replies: 0

Suppose matrices C \in R^{nxk} and X \in R^{nxk} are given, and  it is known that X minimizes the expression


which is the summation of squared differences over all nxn entries.
Can it be proved that such solution X minimizes

Perhaps writing the above in terms of sums could help.

#11 Help Me ! » Interpretation of 3D projection to 2D » 2012-01-09 21:30:25

onako
Replies: 1

Recently I was assigned to implement a procedure involving "3D projections" to "2D space". Supposing that 3D data is stored in a n x 3 matrix X, the final step involves Y=XP, where P is a 3x2 matrix. However, I would need to be sure about the actual meaning of the word "projection" to 2D. In my understanding (I deliberately tend to be descriptive), there are some points in 3D, I'm looking at it from above, and there is a piece of paper below the data (paper is fixed).

So, each time I multiply XP to obtain Y in 2D, it actually means I'm changing the viewpoint, and Y actually corresponds to what I see when looking at the data (as on the fixed paper), right? In other words, with every P, I'm changing the viewpoint and taking the picture (or drawing on a paper, thus 2D) of what I see? If so, that would mean that for any P there is a viewpoint. Please correct me if I'm wrong.

#12 Help Me ! » Find the derivative of this function » 2011-12-27 00:43:50

onako
Replies: 1

Suppose you're given a function


where ecl(x_i, x_j) stands for the Euclidean distance between points x_i and  x_j (1D or 2D).
(s_{ij} and x_j are constants)
How to find the derivative of the above with respect to x_i.
The sum sign and the dimensionality are the problem for me.
Any help on how to solve this is appreciated.

#13 Re: Help Me ! » Specific relation » 2011-04-11 20:06:04

Any suggestions on how to proceed?
Thanks

#14 Re: Help Me ! » Specific relation » 2011-04-07 20:40:29

The relation is guaranteed to work for p=[0, 1]. However, I'm faced with certain extensions. Namely,
given

and   
, can we prove

Note that this is true for p=0.5, but I hope there might be a specific range for p for which the relation works.

#15 Re: Help Me ! » Specific relation » 2011-03-30 02:50:52

I know that for p>1 it does not work. But I wonder if it works for p from the interval [0, 1], meaning any number between 0 and 1; 0.5 for example.
An example where it does not work for this p?

#16 Re: Help Me ! » Specific relation » 2011-03-30 02:35:25

Thanks for the message.
I would appreciate if you could share the reasons.

#17 Help Me ! » Specific relation » 2011-03-30 02:00:49

onako
Replies: 9

Under the assumption

, I'm trying to obtain values of p such that 
.
For p>1 the relation does not work. However, I wonder if it can be proved that it works for p from the interval [0, 1].
I tried to solve this with the nth power of binomial a+b, but the formula I found is complicated.
Is there an easier way to approach the problem?

Thanks

#18 Re: Help Me ! » How to mininize this? » 2011-03-05 00:24:07

Unfortunately, the values of a_i and b_i are different.
Would the expression depend on the largest a_i and b_i dominantly (meaning, the final min value of f(t) mostly depend on some specific a_i and b_i, compared to other a_i and b_i values)?
The function would then look like:

What would now be the value of t for min f(t)? I guess this simplifies the matter

#19 Re: Help Me ! » How to mininize this? » 2011-03-04 02:29:05

Thanks.
But, how do you account for the summation? Also, you're introducing the exponent term in the solution again.
The question is to find t for which f(t) is minimum.

#20 Help Me ! » How to mininize this? » 2011-03-04 00:53:36

onako
Replies: 5

Given a certain function f(x), the value of x for which f(x) is minimum is found through differentiation.
Suppose a function


given the same number of elements in the summation.
So, to discover the right t for which f(t) is minimum, I would need a differentiation.
But, I'm not sure how to proceed given the exponents.
Is differentiation possible here, at all?
Thanks

#21 Re: Help Me ! » Ratio approximation » 2011-02-28 21:41:34

The condition


definitely has influence, but I'm also interested how does the correlation between the a_i and b_i entries
affects the accuracy of the approximation.
Also, other observations might be of help.
Thanks

#22 Help Me ! » Ratio approximation » 2011-02-28 02:34:08

onako
Replies: 1

The following is an approximation problem. Given a ratio:

I wonder what steps to follow to give a reasonable approximation.
This is an intuition

.
Clearly, given that all b_i terms are the same, the approximation is the correct solution.
But, this is not the case. The question is, under which conditions (assumptions on a_i or b_i) is the
above approximation accurate.
I guess, the lower the standard deviation of the b_i's, the more accurate the approximation. But, suppose the
b_i's are [1,2,3,4,5,6,7...]. What condition needs be meet for the accurate approximation.
In a sense, the ratio I'm trying to approximate is the weighted average, and in the approximation I'm discarding
the weights. If this interpretation makes it easier to further interpret, please use it.

Also, as for the conditions, I thought of


How is this condition affecting the accuracy of the ratio approximation. Is the approximation more accurate with
this assumption.
Thanks

#23 Re: Help Me ! » Deviation (specific number) » 2011-02-22 08:34:23

I guess the solution is the weighted average. Or?

#24 Re: Help Me ! » Deviation (specific number) » 2011-02-21 08:58:54

Thanks a lot. I tried to extend the procedure by incorporating the effect of a function f(x)=x^p.
So, I'm trying to incorporate it for individual terms.

Then, extracting the a, b, c terms is not so easy. Perhaps I would need to take different approach.
Thanks.

#25 Re: Help Me ! » Deviation (specific number) » 2011-02-20 22:35:47

Thanks. What would be the relation of D1 and D2 values associated with sets G1 and G2: G1 and G2 having the same length and same standard deviation?
In other words, given two sets G1 and G2 of equal length, what condition needs be satisfied for D1=D2; they would need to have same standard deviation, or same mean?

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