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user44197
  • Member for 10 years, 2 months
  • Last seen more than 9 years ago
53 votes
Accepted

Show that these two numbers have the same number of digits

24 votes
Accepted

A prime of the form $38111111\ldots$

18 votes

simple example of recursive least squares (RLS)

11 votes

Properties of the euler totient function

10 votes

Minimizing the sum of absolute values with a linear solver

10 votes

Continuity $\Rightarrow$ Intermediate Value Property. Why is the opposite not true?

9 votes
Accepted

Prove or disprove: If $A^2$ is normal matrix then $A$ is normal matrix

8 votes
Accepted

Where are the mistakes in the following reasoning?

8 votes

Plotting Primes

8 votes
Accepted

If a matrix commutes with all diagonal matrices, must the matrix itself be diagonal?

8 votes

Prove that $n(n^2 - 1)(n + 2)$ is divisible by $4$ for any integer $n$

8 votes

There exists a power of 2 such that the last five digits are all 3's or 6's. Find the last 5 digits of this number

7 votes
Accepted

Construct a function $f:[0,1] \to [0,1]$ that takes every value in $[0,1]$ an infinite number of times.

7 votes
Accepted

A sequence of functions converging to the Dirac delta

7 votes
Accepted

Proving an infinite number of primes of the form 6n+1

7 votes
Accepted

Evaluating an infinite summation

7 votes

Coordinate-free proof of $\operatorname{Tr}(AB)=\operatorname{Tr}(BA)$?

7 votes
Accepted

A doubt in quadratic inequality

7 votes
Accepted

Does $ST=TS$ with $S,T$ diagonalizable matrices imply that they share eigenspaces?

7 votes

The sum of series involving binomial coefficients

7 votes
Accepted

Solving $f(x+y)-f(x)=yf'\Big(x+ \dfrac y{2}\Big),\forall x,y\in \mathbb R$

6 votes
Accepted

Prove by induction that $P_{n}<2^{2^{n}}$, being $P_{n}$ the $n^{th}$ prime number

6 votes

$(1+1/x)(1+1/y)(1+1/z) = 3$ Find all possible integer values of $x$, $y$, $z$ given all of them are positive integers.

6 votes

Understanding the Handshake Problem

6 votes

The improper integral of $1/(x^2+x)$ from $0$ to $\infty$.

6 votes
Accepted

Find all solutions of ${\frac {1} {x} } + {\frac {1} {y} } +{\frac {1} {z}}=1$, where $x$, $y$ and $z$ are positive integers

5 votes
Accepted

Invertible function $f(x) = \frac{x^3}{3} + \frac{5x}{3} + 2 $

5 votes

trig identity proof help working LHS only

5 votes

Please check my proof and let me know if incorrect.

5 votes
Accepted

If $A$ is invertible, then the $QR$-factorization is unique if we require that the diagonal elements of $R$ are positive.

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