User user44197

53 votes

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Show that these two numbers have the same number of digits

answered Feb 22, 2014 at 3:42

24 votes

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A prime of the form $38111111\ldots$

answered Jan 2, 2014 at 17:09

18 votes

simple example of recursive least squares (RLS)

answered Jan 8, 2014 at 16:11

10 votes

Minimizing the sum of absolute values with a linear solver

answered Dec 31, 2013 at 21:39

10 votes

Properties of the euler totient function

answered Jan 7, 2014 at 8:18

10 votes

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

answered Dec 30, 2013 at 8:22

9 votes

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

answered Dec 28, 2013 at 9:05

8 votes

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

answered Dec 23, 2013 at 4:18

8 votes

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

answered Dec 31, 2013 at 8:41

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

answered Jan 8, 2014 at 1:32

8 votes

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Where are the mistakes in the following reasoning?

answered Feb 2, 2014 at 3:14

8 votes

Plotting Primes

answered Feb 16, 2014 at 7:29

7 votes

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

answered Dec 24, 2013 at 19:53

7 votes

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A doubt in quadratic inequality

answered Dec 25, 2013 at 7:10

7 votes

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

answered Dec 25, 2013 at 22:15

7 votes

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

answered Dec 29, 2013 at 8:18

7 votes

The sum of series involving binomial coefficients

answered Dec 31, 2013 at 6:22

7 votes

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

answered Feb 17, 2014 at 6:09

7 votes

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

answered Feb 11, 2014 at 4:12

7 votes

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A sequence of functions converging to the Dirac delta

answered Feb 4, 2014 at 5:48

7 votes

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Evaluating an infinite summation

answered Jan 13, 2014 at 5:51

6 votes

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

answered Dec 30, 2013 at 7:34

6 votes

Understanding the Handshake Problem

answered Dec 29, 2013 at 4:57

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.

answered Dec 29, 2013 at 7:03

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

answered Dec 23, 2013 at 19:19

6 votes

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

answered Jan 10, 2014 at 2:03

5 votes

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Weird matrix identity

answered Dec 24, 2013 at 6:38

5 votes

How many permutations of the letters in the word MISSISSIPPI are palindromes?

answered Dec 26, 2013 at 5:14

5 votes

convergence of the iterated cosine

answered Dec 30, 2013 at 5:48

5 votes

Accepted

Riemann integral of continuous function is zero implies function is zero

answered Dec 30, 2013 at 3:39

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433 Answers

Show that these two numbers have the same number of digits

A prime of the form $38111111\ldots$

simple example of recursive least squares (RLS)

Minimizing the sum of absolute values with a linear solver

Properties of the euler totient function

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

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

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

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

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

Where are the mistakes in the following reasoning?

Plotting Primes

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

A doubt in quadratic inequality

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

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

The sum of series involving binomial coefficients

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

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

A sequence of functions converging to the Dirac delta

Evaluating an infinite summation

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

Understanding the Handshake Problem

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

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

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

Weird matrix identity

How many permutations of the letters in the word MISSISSIPPI are palindromes?

convergence of the iterated cosine

Riemann integral of continuous function is zero implies function is zero