### Harmonic series without number 9 is convergent

There are tons of proofs that harmonic series diverges. What if we have a harmonic series where we get rid of all the numbers which contains $9$, will it still converge.
For single digit there are $8$ numbers which do not contain number $9$.
Single digit numbers $8$
Two digit numbers $8 \times 9 $
Three digit numbers $8 \times 9 \times 9 = 8\cdot 9^2$
4 digit numbers $latex 8\cdot 9^3$
...
Also notice that largest fraction in harmonic series
Two digits $\frac{1}{10}$
Three digits $\frac{1}{100}$
Four digits $\frac{1}{1000}$
So if we replace the terms of harmonic with these fractions we will get an upper bound on the harmonic series devoid of number $9$
It is $\frac{1}{1}\cdot 8+\frac{1}{10}\cdot 8 \cdot 9+\frac{1}{100}\cdot 8 \cdot 9^2+\cdots+..$ and
$\Rightarrow 8\cdot \frac{1}{1}+8\cdot \frac{9}{10}+8\cdot \frac{9^2}{10^2}+8\cdot \frac{9^3}{10^3}+\cdots$
This is a geometric series and the sum is
$\frac{8}{1-\frac{9}{10}} = 80$.
Thus the geometric series with common ratio of $\frac{9}{10}$ is convergent.

Labels: 9, Convergence, Harmonic series

## Tuesday, February 21, 2017

### At least one of 3 consecutive odd numbers is a multiple of 3

The way to approach this problem is to show that there will always be a number among three consecutive odd numbers that is divisible by $3$.
Let three consecutive odd numbers be $2n+1,2n+3,2n+5$.
If $2n+1$ is a multiple of $3$ then we are done.
Else the remainder when divided by $3$ is either $1$ or $2$. Suppose its $1$ then it means $2x$ is divisible by $3$ which means $2x+3$ is divisible by $3$.
Or suppose the remainder is $2$, which means $2x-1$ is divisible by $3$ which means $2x+2$ and $2x+5$ are divisible by $3$. Hence proved.

Labels: Consecutive odd numbers, Number theory, Prime Number

### Variance of Uniform Distribution

Let the corresponding probabilities are $\frac{1}{n+1}$ for all at the points $\{0,1,2,\cdots,n+1\}$
The general formula is $\sigma^2 = E[x^2]-(E[X])^2$
Let's first calculate $E[X]=0\cdot \frac{1}{n+1}+1\cdot \frac{1}{n+1}+\cdots+n\cdot \frac{1}{n+1}=\frac{0+1+2+\cdots+n}{n+1}=\frac{n(n+1)}{2(n+1)}=\frac{n}{2}$
Now lets calculate $E[X]=0^2\cdot \frac{1}{n+1}+1^2\cdot \frac{1}{n+1}+\cdots+n^2\cdot \frac{1}{n+1}=\frac{0^2+1^2+2^2+\cdots+n^2}{n+1}=\frac{n(n+1)(2n+1)}{6(n+1)}=\frac{n(2n+1)}{6}$
Therefore $\sigma^2 = \frac{n(2n+1)}{6}-\left ( \frac{n}{2}\right )^2 $
$\Rightarrow \frac{n(2n+1)}{6}-\frac{n^2}{4}$
$\Rightarrow \frac{2n(2n+1)-3n^2}{12}$
$\Rightarrow \frac{4n^2+2n-3n^2}{12}$
$\Rightarrow \frac{n^2+2n}{12}=\frac{n(n+2)}{12}$
Now suppose we have same $n+1$ terms shifted from $a$ to $b$ in that case the variance becomes $\frac{(b-a)(b-a+2)}{12}$

Labels: Uniform Distribution, Variance

## Wednesday, January 04, 2017

### Six deceptive problems that no one can solve

This is the title of the article which appeared here

The problems are

1. Twin Prime Conjecture

2. The Moving Sofa Problem

3. The Collatz Conjecture

4. The Beal Conjecture

5. The Inscribed Square Problem

6. Goldbach Conjecture

I am aware of the problems number 1,3 and 6. So I need to investigate the other 3. Which are Moving Sofa, Beal Conjecture and the Inscribed Square Problem.

The problems are

1. Twin Prime Conjecture

2. The Moving Sofa Problem

3. The Collatz Conjecture

4. The Beal Conjecture

5. The Inscribed Square Problem

6. Goldbach Conjecture

I am aware of the problems number 1,3 and 6. So I need to investigate the other 3. Which are Moving Sofa, Beal Conjecture and the Inscribed Square Problem.

Labels: famous unsolved problems

### How many triangular numbers are also Fibonacci Numbers

I heard this from Bruce Edwards that they are only 5 and these are 1,1,3,21, and 55.
I wrote a program in Sagemath to verify that. Here it is
Here is the program with its output

def fibo(n):

fib_1 = 1

fib_2 = 1

if n == 1 or n== 2:

return 1

fib_n=0

for i in range(3,n+1):

fib_n = fib_1+fib_2

fib_1 = fib_2

fib_2 = fib_n

return fib_n

def triNumList(n):

a=[]

for i in range(1,n):

b=i*(i+1)/2

a.append(b)

return a

def fiboList(n):

a=[]

for i in range(1,n):

a.append(fibo(i))

return a

def TriFiboCompare(n):

f = fiboList(n)

t = triNumList(n)

print(f)

print(t)

for i in f:

if i in t:

print("Match",i)

TriFiboCompare(100)

[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368, 75025, 121393, 196418, 317811, 514229, 832040, 1346269, 2178309, 3524578, 5702887, 9227465, 14930352, 24157817, 39088169, 63245986, 102334155, 165580141, 267914296, 433494437, 701408733, 1134903170, 1836311903, 2971215073, 4807526976, 7778742049, 12586269025, 20365011074, 32951280099, 53316291173, 86267571272, 139583862445, 225851433717, 365435296162, 591286729879, 956722026041, 1548008755920, 2504730781961, 4052739537881, 6557470319842, 10610209857723, 17167680177565, 27777890035288, 44945570212853, 72723460248141, 117669030460994, 190392490709135, 308061521170129, 498454011879264, 806515533049393, 1304969544928657, 2111485077978050, 3416454622906707, 5527939700884757, 8944394323791464, 14472334024676221, 23416728348467685, 37889062373143906, 61305790721611591, 99194853094755497, 160500643816367088, 259695496911122585, 420196140727489673, 679891637638612258, 1100087778366101931, 1779979416004714189, 2880067194370816120, 4660046610375530309, 7540113804746346429, 12200160415121876738, 19740274219868223167, 31940434634990099905, 51680708854858323072, 83621143489848422977, 135301852344706746049, 218922995834555169026] [1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 105, 120, 136, 153, 171, 190, 210, 231, 253, 276, 300, 325, 351, 378, 406, 435, 465, 496, 528, 561, 595, 630, 666, 703, 741, 780, 820, 861, 903, 946, 990, 1035, 1081, 1128, 1176, 1225, 1275, 1326, 1378, 1431, 1485, 1540, 1596, 1653, 1711, 1770, 1830, 1891, 1953, 2016, 2080, 2145, 2211, 2278, 2346, 2415, 2485, 2556, 2628, 2701, 2775, 2850, 2926, 3003, 3081, 3160, 3240, 3321, 3403, 3486, 3570, 3655, 3741, 3828, 3916, 4005, 4095, 4186, 4278, 4371, 4465, 4560, 4656, 4753, 4851, 4950] ('Match', 1) ('Match', 1) ('Match', 3) ('Match', 21) ('Match', 55)

def fibo(n):

fib_1 = 1

fib_2 = 1

if n == 1 or n== 2:

return 1

fib_n=0

for i in range(3,n+1):

fib_n = fib_1+fib_2

fib_1 = fib_2

fib_2 = fib_n

return fib_n

def triNumList(n):

a=[]

for i in range(1,n):

b=i*(i+1)/2

a.append(b)

return a

def fiboList(n):

a=[]

for i in range(1,n):

a.append(fibo(i))

return a

def TriFiboCompare(n):

f = fiboList(n)

t = triNumList(n)

print(f)

print(t)

for i in f:

if i in t:

print("Match",i)

TriFiboCompare(100)

[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368, 75025, 121393, 196418, 317811, 514229, 832040, 1346269, 2178309, 3524578, 5702887, 9227465, 14930352, 24157817, 39088169, 63245986, 102334155, 165580141, 267914296, 433494437, 701408733, 1134903170, 1836311903, 2971215073, 4807526976, 7778742049, 12586269025, 20365011074, 32951280099, 53316291173, 86267571272, 139583862445, 225851433717, 365435296162, 591286729879, 956722026041, 1548008755920, 2504730781961, 4052739537881, 6557470319842, 10610209857723, 17167680177565, 27777890035288, 44945570212853, 72723460248141, 117669030460994, 190392490709135, 308061521170129, 498454011879264, 806515533049393, 1304969544928657, 2111485077978050, 3416454622906707, 5527939700884757, 8944394323791464, 14472334024676221, 23416728348467685, 37889062373143906, 61305790721611591, 99194853094755497, 160500643816367088, 259695496911122585, 420196140727489673, 679891637638612258, 1100087778366101931, 1779979416004714189, 2880067194370816120, 4660046610375530309, 7540113804746346429, 12200160415121876738, 19740274219868223167, 31940434634990099905, 51680708854858323072, 83621143489848422977, 135301852344706746049, 218922995834555169026] [1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 105, 120, 136, 153, 171, 190, 210, 231, 253, 276, 300, 325, 351, 378, 406, 435, 465, 496, 528, 561, 595, 630, 666, 703, 741, 780, 820, 861, 903, 946, 990, 1035, 1081, 1128, 1176, 1225, 1275, 1326, 1378, 1431, 1485, 1540, 1596, 1653, 1711, 1770, 1830, 1891, 1953, 2016, 2080, 2145, 2211, 2278, 2346, 2415, 2485, 2556, 2628, 2701, 2775, 2850, 2926, 3003, 3081, 3160, 3240, 3321, 3403, 3486, 3570, 3655, 3741, 3828, 3916, 4005, 4095, 4186, 4278, 4371, 4465, 4560, 4656, 4753, 4851, 4950] ('Match', 1) ('Match', 1) ('Match', 3) ('Match', 21) ('Match', 55)

Labels: fibonacci, sagemath, triangular numbers

## Tuesday, January 03, 2017

### Tiling a deleted 64 grid using dominos

This is a classic problem that can be found in most books on problem solving and I re-encountered on my reading of Mathematics a Very Short Introduction by Timothy Gower. The problem is if you delete the two diagonal squares of a 64 grid squares like a chess board and then you have to tile it using dominos.
The way to do is to think of is to realize that each domino occupies a black and white square. Assuming that the two black squares were deleted. There are now 62 squares in total out of which 32 are white and 30 are black. So no matter how we tile. We will be left with 2 white squares and since a domino tiles only black and white those two will remain untiled.

Labels: Gower, Problem Solving

## Friday, December 23, 2016

### Trials until first success

This is a famous problem in 50 challenging problems in probability. The question is
On the average, how many times must a die be thrown until one gets a 6 ?
Let p be the probability of getting a 6. Now we can get this at first trial with probability $p$, on 2nd trial with probability $(1-p)p$, or on 3 trial with $(1-p)^2p$ and so on.
The expected value is $N= 1p+2(1-p)p+3(1-p)^2p+4(1-p)^3p+\cdots$
$\Rightarrow N(1-p)=1(1-p)p+2(1-p)^2p+3(1-p)^3p+4(1-p)^4p+\cdots$
Subtracting we get
$\Rightarrow N(1-1+p)=p+(1-p)p+(1-p)^2p+(1-p)^3p+\cdots$
$\Rightarrow Np= p((1-p)+(1-p)^2+(1-p)^3+(1-p)^4+\cdots$
$\Rightarrow N= \frac{1}{1-(1-p)}$
$\Rightarrow N= \frac{1}{p}$
Thus if the probability of success is $p$. On an average it will take $\frac{1}{p}$ number of trials for the first success.

Labels: Mosteller, Probability

## Saturday, December 10, 2016

### Factitious, Facetious and Circumspect

Factitious (adjective):

Made or manufactured; not natural

Made up in the sense of contrived; a sham, fake or phony

The CIA agent hid his message inside the hollow factitious rock by the bridge; his handler would pick up the message a few hours later.

My dad's factitious smile didn't fool anyone; he was definitely not happy to see our cousins show up once again unannounced.

Facetious

Circumspect (adjective)

Cautious, Prudent

Circumspect is a combination of circum ("around") and spect ("look"). To remember this word, think of a cautious person "looking around" before he or she acts.

Made or manufactured; not natural

Made up in the sense of contrived; a sham, fake or phony

The CIA agent hid his message inside the hollow factitious rock by the bridge; his handler would pick up the message a few hours later.

My dad's factitious smile didn't fool anyone; he was definitely not happy to see our cousins show up once again unannounced.

Facetious

Circumspect (adjective)

Cautious, Prudent

Circumspect is a combination of circum ("around") and spect ("look"). To remember this word, think of a cautious person "looking around" before he or she acts.

### Factotum and Procrustean

Two new words
Factotum: A factotum is someone hired to do a variety of jobs, someone who has many responsibilities, a jack of all trade.

Example: Tessa, the office factotum, does the billing, answers the phones, helps out in the PR department, and even knows how to cook a mean blueberry scone- she's indispensable. Andrea in my school is a factotum. "fac" is a latin word for facio, meaning to make or do. "totum" means all. Therefore factotum is someone who does all. Other example: The intendant became the king's factotum. Gallop off to Texas, he said to the factotum who appeared at his call.

Procrustean: Procustes was a mythical bandit of Attica who would waylay hapless travelers and attempt to fit them to his iron bed. If travelers were too long for the bed, he'd cut off their feet. If they were too short, he'd stretch them out. A procrustean bed has come to mean an arbitrary standard to which something is forced to confirm.

Even though student's poem unanimously won all county writing contest, the procrustean English teacher gave her an F for failing to do the i in her name.

I was shocked by their procrustean attitude towards completing the syllabus. I think a teacher is a best judge of determining the pace of the class.

Example: Tessa, the office factotum, does the billing, answers the phones, helps out in the PR department, and even knows how to cook a mean blueberry scone- she's indispensable. Andrea in my school is a factotum. "fac" is a latin word for facio, meaning to make or do. "totum" means all. Therefore factotum is someone who does all. Other example: The intendant became the king's factotum. Gallop off to Texas, he said to the factotum who appeared at his call.

Procrustean: Procustes was a mythical bandit of Attica who would waylay hapless travelers and attempt to fit them to his iron bed. If travelers were too long for the bed, he'd cut off their feet. If they were too short, he'd stretch them out. A procrustean bed has come to mean an arbitrary standard to which something is forced to confirm.

Even though student's poem unanimously won all county writing contest, the procrustean English teacher gave her an F for failing to do the i in her name.

I was shocked by their procrustean attitude towards completing the syllabus. I think a teacher is a best judge of determining the pace of the class.

## Sunday, December 04, 2016

### Locks in a grid problem

This is a simulation to an interesting problem about a lock. Which has n key slots. These slots are either vertical or horizontal. To open the lock these slots have to be in a particular configuration.
If we put the key in one slot and turn its orientation all the other slots in that particular row and column change their configuration. Is it possible to open this lock for a particular configuration of 16 slots ? If yes then how ?

` ````
"""The purpose of this is to simulate the problem of locks
where when you turn on the key in the grid all the other
keys in the same row and column also turn. I represent this
by 0 and 1
One can experiment and learn that if the grid is even by even
then we can change any particular bit by"""
n = 4
a=[]
sum = 0
# It prints the whole Grid of Locks
def printMatrix(val):
for i in range(n):
print(val[i])
# It flips the entries from 0 to 1 and 1 to 0
def flip(val):
if val == 0:
return 1
return 0
# It changes the entries in a given row and column
def ChangeRowChangeColumn(a,i,j):
a[i][j]=flip(a[i][j])
for k in range(n):
a[i][k]=flip(a[i][k])
for k in range(n):
a[k][j]=flip(a[k][j])
for i in range(1,n+1):
b=[]
for j in range(1,n+1):
b.append(1)
c = b[:]
sum= sum+n
a.append(c)
del b[:]
printMatrix(a)
flag=1
while(flag == 1):
row=int(input("Enter what row you want to change ?"))
col=int(input("Enter what column you want to change?"))
ChangeRowChangeColumn(a,row,col)
printMatrix(a)
flag = int(input("Enter 0 or 1 to continue"))
```

Labels: lock problem, Python