14,557 reputation
11938
bio website math.brown.edu/~lubinj
location Minnesota
age 78
visits member for 3 years, 1 month
seen 16 hours ago

Aged mathematician


Nov
2
comment How to determine if set of vectors is a basis for W
This isn’t right. See @LuisValerin’s response for the right approach.
Oct
31
comment Is decresing intersection of closure of connected set is connected?
Good counterexample.
Oct
30
comment Help me find $S_{-2}$ using the polynomial
Did you notice that the product of the three roots of your second polynomial is $1$?
Oct
30
awarded  p-adic-number-theory
Oct
29
awarded  Nice Answer
Oct
29
answered How do you solve a logarithm with a non-integer base?
Oct
29
answered Solution of Pell equation over field of p-adic numbers
Oct
29
answered Calculate the power series
Oct
28
comment Rules for Calculating Modulo
I like this approach — thanks!
Oct
28
answered Rules for Calculating Modulo
Oct
28
comment Example of a homomorphism with a right or left inverse function that its right or left inverse is not a homomorphism
Take the surjective homomorphism $\mathbb Z\to\mathbb Z/(n)$, $n\ne0$.
Oct
28
comment speaking about math
The one exception is when there’s a blind person in the audience. Then perhaps it might be wise to spell it all out. And of course there’s Victor Borge’s method, where each punctuation mark has its own sound. This could be adapted to mathematics, I’m sure.
Oct
28
comment Line-of-Sight Angle on Sphere
@John, you’re right on the mark. Imagine a great-circle path that starts on the equator, maybe in a NE direction, to hit the equator again at the antipode of the original point. One’s heading starts at $45^\circ$, and at the midpoint is due E, $90^\circ$, while at the end of the trip halfway round the world, you’re heading SE, $135^\circ$.
Oct
28
comment Line-of-Sight Angle on Sphere
I’m not sure I understand, but it seems to me that the direction in which you have to look lies somewhere in the plane determined by you, the launch point, and the center of the earth, in other words the vertical plane containing you and the launch point. This hits the earth’s surface along the great-circle path that you know how to compute, so that the azimuth is exactly what’s given by that method. The altitude is another issue.
Oct
28
comment Every subgroup $H$ of a group $G$ is a union of cyclic subgroups of $G$
Isn’t every element of $G$ in a cyclic subgroup?
Oct
27
answered Use of the concept of subgroup vs field extension
Oct
26
comment Use of the concept of subgroup vs field extension
Have to leave for a few hours. I’ll try to think up something good in that period, and will make an attempt to give a full answer.
Oct
25
comment Use of the concept of subgroup vs field extension
@PraphullaKoushik, I think that’s a lot of it. Also, if we ask what fields contain a given $k$, we can prove a number of interesting and useful theorems, while the universe of groups containing a given group $G$ is much wilder, and for me much less interesting.
Oct
25
comment Use of the concept of subgroup vs field extension
I think you’re saying that in the case of groups, we emphasize the substructures, while for fields, we emphasize the overlying structures. This is true enough; perhaps because among the most interesting fields we find those that have no proper subfields.
Oct
25
comment Roots Of An Inseparable Polynomial.
That’s the only root there is. That’s why it’s called inseparable.