67,948 reputation
143129
bio website math.ethz.ch/~blatter
location Switzerland
age 79
visits member for 4 years, 3 months
seen 20 mins ago

2d
comment When is a vector field on a manifold restricted to a submanifold $X$ a vector field on $X$?
To put it simply: The condition is that $V(p)\in T_p(X)$ for all $p\in X$.
2d
answered Find the indicial equation
2d
answered Combinations in application - “smooth order”
Nov
24
answered Showing that $|f_ng_n-fg|<\epsilon$, where $f_n\to f, g_n\to g$
Nov
24
answered then $g$=$f$ in a hilbert space
Nov
24
comment necessary and sufficient conditions in ODE theory
In the theory of ODEs it's about "existence and uniqueness", not "necessity and sufficiency". The theory provides sufficient conditions for the existence and uniqueness of solutions. It would be difficult to write down interesting "necessary" conditions.
Nov
24
answered Simple problem about Laplace Equation in a domain
Nov
23
revised Matrix tranformation, how to
added 6 characters in body
Nov
23
answered Showing that Lebesgue measure is preserved by translations of the $d$-dimensional torus
Nov
22
revised Find the locus of points M the difference of the squares of whose distances from two given points A and B is equal to a given value c.
added 512 characters in body
Nov
22
revised Invent transformation mapping ellipsoid to unit sphere
added 254 characters in body
Nov
22
revised Showing that Lebesgue measure is preserved by translations of the $d$-dimensional torus
edited title
Nov
22
answered Matrix tranformation, how to
Nov
22
comment Find the locus of points M the difference of the squares of whose distances from two given points A and B is equal to a given value c.
You are on the wrong track. Perform the same calculations as you did in your first approach, but using the disposition suggested in my answer. Then interpret the resulting condition on $P=(x,y)$ in words.
Nov
22
answered Find the locus of points M the difference of the squares of whose distances from two given points A and B is equal to a given value c.
Nov
21
answered Invent transformation mapping ellipsoid to unit sphere
Nov
21
revised How long is the curve that a creature walks?
added 214 characters in body
Nov
21
comment List of powers of Natural Numbers
What does "do s[i] += s[i-1]" mean?
Nov
21
answered How long is the curve that a creature walks?
Nov
21
comment Lesser known derivations of well-known formulas and theorems
Here is a version of Fuerstenberg's proof: We are arguing about periodic sets of integers. The set $N_p$ of all integers prime to $p$ is periodic, and the intersection of two periodic sets is periodic. If there were only finitely many primes the set $\{1,-1\}$ would be periodic.