For questions about mathematical induction, a method of mathematical proof. Mathematical induction generally proceeds by proving a statement for some integer, called the *base case*, and then proving that if it holds for one integer then it holds for the next integer. This tag is primarily aimed ...

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Proving $1^3+ 2^3 + \cdots + n^3 = \left(\frac{n(n+1)}{2}\right)^2$ using induction

How can I prove that $$1^3+ 2^3 + \cdots + n^3 = \left(\frac{n(n+1)}{2}\right)^2$$ for all $n \in \mathbb{N}$? I am looking for a proof using mathematical induction. Thanks
29
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5answers
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Dominoes and induction, or how does induction work?

I've never really understood why math induction is supposed to work. You have these 3 steps: Prove true for base case (n=0 or 1 or whatever) Assume true for n=k. Call this the induction ...
22
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5answers
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Why $a^n - b^n$ is divisible by $a-b$?

I did some mathematical induction problems on divisibility $9^n$ $-$ $2^n$ is divisible by 7. $4^n$ $-$ $1$ is divisible by 3. $9^n$ $-$ $4^n$ is divisible by 5. Can these be generalized as $a^n$ ...
5
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3answers
387 views

Given $n \in \mathbb N$, prove $\sum^n_{k=0}(-1)^k {n \choose k} = 0$

I tried to solve it using induction, but that got me no were, in the middle of the equation stat appearing ks that I don't see how to get out of the equation. I think the easiest way to prove it, it's ...
53
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6answers
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Induction on Real Numbers

One of my Fellows asked me whether total induction is applicable to real numbers, too ( or at least all real numbers ≥ 0) . We only used that for natural numbers so far. Of course you have to change ...
3
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5answers
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Proof that $n^2 < 2^n$

How do I prove the following statement by induction? $$n^2 \lt 2^n$$ $P(n)$ is the statement $n^2 \lt 2^n$ Claim: For all $n \gt k$, where $k$ is any integer, $P(n)$ (since $k$ is any integer, I ...
7
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2answers
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Inductive proof that ${2n\choose n}=\sum{n\choose i}^2.$

I would like to prove inductively that $${2n\choose n}=\sum_{i=0}^n{n\choose i}^2.$$ I know a couple of non-inductive proofs, but I can't do it this way. The inductive step eludes me. I tried naively ...
10
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4answers
2k views

What is the second principle of finite induction?

I understand the principle of finite induction, but my book then mentions that there is a variant of the first where requirement b is changed to If k is a positive integer such that 1,2...,k ...
9
votes
4answers
883 views

Induction Proof that $x^n-y^n=(x-y)(x^{n-1}+x^{n-2}y+\ldots+xy^{n-2}+y^{n-1})$

This question is from [Number Theory George E. Andrews 1-1 #3]. Prove that $$x^n-y^n=(x-y)(x^{n-1}+x^{n-2}y+\ldots+xy^{n-2}+y^{n-1}).$$ This problem is driving me crazy. $$x^n-y^n = ...
3
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2answers
1k views

Proof the inequality $n! \geq 2^n$ by induction

I'm having difficulity solving an exercise in my course. The question is: Prove that $n!\geq 2^n$. We have to do this with induction. I started like this: The lowest natural number where the ...
11
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6answers
3k views

Show that $11^{n+1}+12^{2n-1}$ is divisible by $133$.

Problem taken from a paper on mathematical induction by Gerardo Con Diaz. Although it doesn't look like anything special, I have spent a considerable amount of time trying to crack this, with no luck. ...
3
votes
5answers
364 views

Proving the total number of subsets of S is equal to $2^n$

Student here! Just reading Liebecks Introduction to pure mathematics for fun and I made an attempt at proving the total number of subsets of S is equal to $2^n$. I realized that the total number of ...
11
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6answers
2k views

Proving $\sum\limits_{k=1}^{n}{\frac{1}{\sqrt{k}}\ge\sqrt{n}}$ with induction

I am just starting out learning mathematical induction and I got this homework question to prove with induction but I am not managing. $$\sum\limits_{k=1}^{n}{\frac{1}{\sqrt{k}}\ge\sqrt{n}}$$ ...
4
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2answers
751 views

Induction proof concerning a sum of binomial coefficients: $\sum_{j=m}^n\binom{j}{m}=\binom{n+1}{m+1}$

I'm looking for a proof of this identity but where j=m not j=0 http://www.proofwiki.org/wiki/Sum_of_Binomial_Coefficients_over_Upper_Index $$\sum_{j=m}^n\binom{j}{m}=\binom{n+1}{m+1}$$
2
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3answers
108 views

How to prove this statement: $\binom{r}{r}+\binom{r+1}{r}+\cdots+\binom{n}{r}=\binom{n+1}{r+1}$

Let $n$ and $r$ be positive integers with $n \ge r$. Prove that Still a beginner here. Need to learn formatting. I am guessing by induction? Not sure what or how to go forward with this. Need help ...
33
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18answers
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Examples of mathematical induction

What are the best examples of mathematical induction available at the secondary-school level---totally elementary---that do not involve expressions of the form $\bullet+\cdots\cdots\cdots+\bullet$ ...
18
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6answers
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Induction: $\sum_{k=0}^n \binom nk k^2 = n(1+n)2^{n-2}$

I found crazy (for me at least) induction example, in fact it just would be nice to prove. (Even have problems with starting) Any hints are highly valued: ...
10
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1answer
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What's the difference between simple induction and strong induction?

I just started to learn induction in my first year course. I'm having a difficult time grasping the concept. I believe I understand the basics but could someone explain the summary of simple induction ...
3
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4answers
2k views

Proving the sum of the first $n$ natural numbers by induction [duplicate]

I am currently studying proving by induction but I am faced with a problem. I need to solve by induction the following question. $$1+2+3+\ldots+n=\frac{1}{2}n(n+1)$$ for all $n > 1$. Any ...
2
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5answers
3k views

Proof by induction that $ \sum_{i=1}^n 3i-2 = \frac{n(3n-1)}{2} $

I'm starting to understand how induction works (with the whole $k \to k+1$ thing), but I'm not exactly sure how summations play a role. I'm a bit confused by this question specifically: $$ ...
44
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2answers
3k views

Proof of 1 = 0 by Mathematical Induction on Limits?

I got stuck with a problem that pop up in my mind while learning limits. I am still a high school student. Define $P(m)$ to be the statement: $\quad ...
21
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5answers
847 views

How does one actually show from associativity that one can drop parentheses?

I've always heard this reasoning, and it makes obvious sense, but how do you actually show it for some arbitrary product? Would it be something like this? ...
5
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2answers
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Strong Mathematical Induction: Why More than One Base Case?

I am trying to understand this example of strong induction. I know normal induction. In normal induction, if base case is true then we assume some number $n$ to be true. Afterwards, we prove $n+1$ is ...
7
votes
3answers
900 views

Prove by induction Fibonacci equality

[question:] Prove by induction that the i th Fibonacci number satisfies the equality $$F_i=\frac{\phi^i-\hat{\phi^i}}{\sqrt5}$$where $\phi$ is the golden ratio and $\hat{\phi}$ is its conjugate. ...
3
votes
2answers
194 views

What is the formula for $1/(1\cdot 2)+1/(2\cdot 3)+1/(3\cdot 4)+\ldots +1/(n(n+1))$

How can I find the formula for the following equation? $$\frac{1}{1\cdot 2}+\frac{1}{2\cdot 3}+\frac{1}{3\cdot 4}+\ldots +\frac{1}{n(n+1)}$$ More importantly, how would you approach finding the ...
3
votes
2answers
7k views

Proving formula for product of first n odd numbers

I have this formula which seems to work for the product of the first n odd numbers (I have tested it for all numbers from $1$ to $100$): $$\prod_{i = 1}^{n} (2i - 1) = \frac{(2n)!}{2^{n} n!}$$ How ...
4
votes
3answers
694 views

induction proof: $\sum_{k=1}^nk^2 = \frac{n(n+1)(2n+1)}{6}$

I encountered the following induction proof on a practice exam for calculus: $$\sum_{k=1}^nk^2 = \frac{n(n+1)(2n+1)}{6}$$ I have to prove this statement with induction. Can anyone please help me ...
2
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5answers
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Prove by mathematical induction that $2n ≤ 2^n$, for all integer $n≥1$?

I need to prove $2n \leq 2^n$, for all integer $n≥1$ by mathematical induction? This is how I prove this: Prove:$2n ≤ 2^n$, for all integer $n≥1$ Proof: $2+4+6+...+2n=2^n$ $i.)$ Let $P(n)=1 ...
1
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2answers
844 views

Proving $\sum\limits_{i=0}^n i 2^{i-1} = (n+1) 2^n - 1$ by induction

I'm trying to apply an induction proof to show that $((n+1) 2^n - 1 $ is the sum of $(i 2^{i-1})$ from $0$ to $n$. the base case: L.H.S = R.H.S we assume that $(k+1) 2^k - 1 $ is true. we need to ...
0
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2answers
75 views

Proving the geometric sum formula by induction

$$\sum_{k=0}^nq^k = \frac{1-q^{n+1}}{1-q}$$ I want to prove this by induction. Here's what I have. $$\frac{1-q^{n+1}}{1-q} + q^{n+1} = \frac{1-q^{n+1}+q^{n+1}(1-q)}{1-q}$$ I wanted to factor a ...
6
votes
4answers
2k views

How does backwards induction work to prove a property for all naturals?

I was reading a blogpost here: http://mzargar.wordpress.com/2009/07/19/cauchys-method-of-induction/ One thing that threw me off was that after the first four large displayed equations, there is the ...
6
votes
4answers
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Prove that $1^3 + 2^3 + … + n^3 = (1+ 2 + … + n)^2$ [duplicate]

This is what I've been able to do: Base case: $n = 1$ $L.H.S: 1^3 = 1$ $R.H.S: (1)^2 = 1$ Therefore it's true for $n = 1$. I.H.: Assume that, for some $k \in \Bbb N$, $1^3 + 2^3 + ... + k^3 = (1 ...
6
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4answers
185 views

Induction proof: $S$ contains powers of 2 and predecessors implies $S={\bf N}$

Let $S$ be a subset of $\mathbb{N}$ such that $$2^k\in S\quad\forall\ k\in\mathbb N$$ and, $$k\in S\Rightarrow k-1\in S\quad\forall\ k\ge2,k\in\mathbb N$$ Show by induction that $S=\mathbb N$.
5
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5answers
762 views

Prove that $(a-b) \mid (a^n-b^n)$

I'm trying to prove by induction that for all $a,b \in \mathbb{Z}$ and $n \in \mathbb{N}$, that $(a-b) \mid (a^n-b^n)$. The base case was trivial, so I started by assuming that $(a-b) \mid (a^n-b^n)$. ...
4
votes
4answers
742 views

Prove by Mathematical Induction: $1(1!) + 2(2!) + \cdot \cdot \cdot +n(n!) = (n+1)!-1$

Prove by Mathematical Induction . . . $1(1!) + 2(2!) + \cdot \cdot \cdot +n(n!) = (n+1)!-1$ I tried solving it, but I got stuck near the end . . . a. Basis Step: $(1)(1!) = (1+1)!-1$ $1 = ...
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3answers
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Prove $(n^5-n)$ is divisible by 5 by induction.

So I started with a base case $n = 1$. This yields $5|0$, which is true since zero is divisible by any non zero number. I let $n = k >= 1$ and let $5|A = (k^5-k)$. Now I want to show $5|B = ...
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2answers
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How to prove $a^n < n!$ for all $n$ sufficiently large, and $n! \leq n^n$ for all $n$, by induction?

I have a couple things I want to prove. I'm pretty sure a proof by induction is the best route for these. First, I need to show that $5^n < n!$ from some $n_{0} > 0$. I'm choosing $n_{0} = ...
4
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2answers
335 views

Representing Any $n \geq 4$ as a Sum of 2's and 5's

Use induction on $n$ to prove that for all integers $n\geq 4$, postage of $n$ cents can be realized using only $2$ cent and $5$ cent stamps. I thinks it is little bit different. How can I use ...
0
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2answers
931 views

Help with graph induction question?

Given a graph $G$ with $n$ vertices, where $n$ is even, prove by induction that if every vertex has degree $n/2 + 1$, then $G$ must contain a 3-cycle. A 3-cycle is a set of 3 vertices, $a; b; c$ such ...
0
votes
5answers
166 views

Prove by induction: $2^n = C(n,0) + C(n,1) + \cdots + C(n,n)$ [duplicate]

This is a question I came across in an old midterm and I'm not sure how to do it. Any help is appreciated. $$2^n = C(n,0) + C(n,1) + \cdots + C(n,n).$$ Prove this statement is true for all $n ...
25
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7answers
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Is there no solution to the blue-eyed islander puzzle?

Text below copied from here The Blue-Eyed Islander problem is one of my favorites. You can read about it here on Terry Tao's website, along with some discussion. I'll copy the problem here as ...
13
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2answers
831 views

9 pirates have to divide 1000 coins…

A band of 9 pirates have just finished their latest conquest - looting, killing and sinking a ship. The loot amounts to 1000 gold coins. Arriving on a deserted island, they now have to split up the ...
5
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5answers
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Difficulties in a proof by mathematical induction (involves evaluating $\sum r3^r$).

Please help. I've been stuck on this for 2 days. Haven't found any easy explaining text. The question is : Prove by mathematical induction that : $$ \sum_{r=1}^n r3^r = \frac{3}{4} \left[ 3^n \left( ...
13
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2answers
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What is complete induction, by example? $4(9^n) + 3(2^n)$ is divisible by 7 for all $n>0$

So, I've been revising for an exam and I came up against the question " prove $4(9^n) + 3(2^n)$ is divisible by 7 for all $n>0$. Now, I know how to do this. If I assume $n=k$ divisible by $7$, ...
12
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11answers
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Proof that $n^3+2n$ is divisible by 3

I'm trying to freshen up for school in another month, and I'm struggling with the simplest of proofs! Problem: For any natural number n , n3 + 2n is divisible by 3. This makes sense ...
5
votes
3answers
710 views

Fake induction proof

Using the induction method: $(\forall P)[[P(0) \land ( \forall k \in \mathbb{N}) (P(k) \Rightarrow P(k+1))] \Rightarrow ( \forall n \in \mathbb{N} ) [ P(n) ]]$ Why this proof is wrong? $P(x)\equiv ...
3
votes
4answers
330 views

Proving that there are infinite cardinal numbers >$\mathfrak{c}$

I was reading Simmons' book and he states that there are infinite cardinal numbers > $\mathfrak{c}$ where $\mathfrak{c}$ denotes the number of Real Numbers. For this, he states that we can construct ...
7
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2answers
8k views

How do I prove this by induction? (sum of powers of 2)

How do I prove this by induction? Prove that for every natural number n, $ 2^0 + 2^1 + ... + 2^n = 2^{n+1}-1$ Here is my attempt. Base Case: let $ n = 0$ Then, $2^{0+1} - 1 = 1$ Which is true. ...
4
votes
3answers
447 views

Proof by induction or contradiction that $(4k + 3) ^2 - (4k + 3)$ is not divisible by $4$?

I have to prove that $(4k + 3) ^2 - (4k + 3)$ is not divisible by $4$. What would be the best approach for this, proof by induction or contradiction? I've tried both and haven't got very far. Any ...
4
votes
2answers
4k views

Compute $1^2 + 3^2+ 5^2 + \cdots + (2n-1)^2$ by mathematical induction

I am doing mathematical induction. I am stuck with the question below. The left hand side is not getting equal to the right hand side. Please guide me how to do it further. $1^2 + 3^2+ 5^2 + ...