Mathematical Induction-II

Author

Abhirup Moitra

Published

September 27, 2023

“Eliminate all other factors, and the one which remains must be the truth.” ~Sherlock Holmes (The Sign of Four)

In the previous episode of Mathematical Induction-I, we discussed the philosophy of mathematical induction, introduced the basic idea of mathematical induction, and solved some problems. In this episode of mathematical induction first, we’ll recall the concept of mathematical induction and then we’ll discuss the more interesting problem and their solution strategies.

Recalling The Philosophy of Mathematical Induction

Domino Toppling

Mathematical induction is like a domino effect. The domino effect is a chain reaction that occurs when one event sets off a series of similar, related, or connected events. It is a reference to a series of standing dominoes, each of which topples the next, creating a chain reaction.

You may be familiar with the term called ‘Domino Toppling’. Domino toppling is achieved by standing dominoes on end and arranging them in the desired patterns and sequences. Such a sequence is called a domino run and then triggers the first one in line to create a chain reaction also called the domino effect. Mathematical Induction uses this principle to prove certain properties.

Mathematical & Logical Perspective

Let Pn be a property that depends on a certain number n. For example,Pn:Tile numbern fallsThe steps will prove by induction are the following.

Pn:Tile numbernfalls

  1. Check P0 is true. ( The first tile falls.)

  2. nNn0 assumes Pn is true and proves Pn+1 also true. (if a tile falls it pushes the next. )

Here step 1 is called the base case and step 2 is the induction step. After completing these two steps we can conclude that Pn is true for all natural numbers n.

Now a question can arise How come we assume that Pn is true when this is what we want to prove?

The answer i.e., there are two completely different things nN,Pn is true, meaning all the tiles fall which is our final target. On the other side, we have nN,PnPn+1 also true, which means that if a tile falls it causes the next one to fall. This does not necessarily means that the tile number n falls.

More generally saying that AB we are not saying that A is true or B is true. We are just saying if ever A is true then B is also true or the implication is true.

In the case of dominoes, we do not need to push a tile to see if it causes the next one to fall. All we need to prove is that the tiles are close enough so that the implication and therefore the induction step is satisfied. If the first tile falls and the distance between the tiles is small then all

the tiles fall or more formally P0 and nN,PnPn+1 together imply that nN,Pn.

Now, we’ll discuss a few special examples to help you better understand mathematical induction

Problem Solving

Objective Type Question

Q1. Prove using mathematical induction that Pn:2nn+5 nN

Pn:2nn+5

Pn+1:2n+1n+6

IfPnis true, means2nn+52n+12n+10n+6

2n+1n+6nN

Therefore, the induction step is valid for any natural number n. So, we can claim that,

PnPn+1

Hence, Pn:2nn+5 nN is true.

The domino tiles are close to each other. However neither P0,P1 norP2 is true. The above property is true only starting from 3. The base case here is 3. Adding the induction step we conclude that Pn is true only n3. Tile 0,1 and 2 remain upright the others fall one after the other starting from 3.

Q2. Show that the property holds Pn:nRQ

Here, Pn:nRQ means n belongs to RQ where RQ={x|xR,xQ} i.e., n is an irrational number. If n is not rational, then adding 1 won’t make the result rational. Therefore, n+1 also irrational.

nRQ(n+1)RQ Therefore, Pn does imply Pn+1.

Try Yourself

  1. Prove using mathematical induction that 2n5n8

  2. Prove using mathematical induction that n+12n4

Subjective Type of Question

Dealing Inequality Condition

  1. Prove that(2n)!(n!)2>4n2n+1nN

    Solution:

    Consider that n is a natural number and given that Pn is true

    Pn:(2n)!(n!)2>4n2n+1

Now,n=1then, L.H.S = 2!(1!)2=2and R.H.S =42+1=43L.H.S > R.H.S

P1 is true

Let us considermis a natural number and;Pmis true.

(2m)!(m!)2>4m2m+1(1)

We will prove that, Pm+1is also true.

{2(m+1)}!{(m+1)!}2=(2m+2)!{(m+1)m!}2=(2m+2)(2m+1).(2m)!(m+1)2(m!)2

=2(2m+1)(m+1).(2m)!m!>2(2m+1)(m+1).4m2m+1(Getting from the (1))

=8mm+1(2)

Now,8mm+14(m+1)2(m+1)+1=4[2mm+1m+12m+3]=4(3m2+4m1)(m+1)(2m+3)>0

8mm+1>4(m+1)2(m+1)+1

Getting from (2){2(m+1)}!{(m+1)!}2>4(m+1)2(m+1)+1

Pm+1is true whenPmis true.

(2n)!(n!)2>4n2n+1nN(Q.E.D)

  1. Prove that 1+2+3++n<nn(n+1)2

    Solution:

    Pn:1+2+3++n<nn(n+1)2

    From the above assertion Pn is true for n=2. Now, assume assertion for m i.e.,

    Pm:1+2+3++m<mm(m+1)2 As, m goes to m+1 then L.H.S increased by m+1 while the R.H.S increased by

    m+1<[(m+1)(m+1)(m+2)2mm(m+1)2 ](1)

    1<[(m+1)(m+2)2mm2](2)

    1<(m+2)2+m[(m+2)2m2](3)

    Symbolizing the expression in (3) above as 1<A+m(B), clearly A>1 and B>0. Therefore, the inequality asserted in (2) above is established.

    This concludes the induction step. Hence, the original assertion is true, by induction.

  2. Prove that (1+x)n>1+nxwherenZ+andn2;x>1.

    Solution:

Pn:(1+x)n>1+nx, x>1

Ifn=2,(1+x)n=(1+x)2=1+2x+x2>1+2x[because,x2>0]  P2is true.

Let us assume,mis a positive integer andPmis true whenm2

(1+x)m>1+mx(1)

We will prove thatPm+1is true i.e.,(1+x)m+1>1+(m+1)x. Nowx>1x+1>0.

Multiplying by1+xboth side of (1) and we get;

(1+x).(1+x)m>(1+x)(1+mx)

(1+x)m+1>1+mx+x+mx2=1+(m+1)x+mx2

(1+x)m+1>1+(m+1)x[ mx2>0]

Pm+1is true. Hence,Pmis true. So thatPm+1is true wherem2is a positive integer.

Therefore, mathematical induction step proves that, forn2and fornZ+Pnis true.

(1+x)n>1+nxwherenZ+andn2;x>1.(Proved)

  1. For which n the inequality 2n>2n+1 holds?

Solution: Let us assume that n is a natural number and the given inequality can be written as,

Pn:2n>2n+1nN. Now n=1 2n=2 and 2n+1=2.1+1=3,  In this case 2n>2n+1 does not hold.

Again, for n=2 then 22=4 and 2n+1=2.2+1=5 In this case 2n>2n+1 does not hold.

For, n=3 2n=23=8 and 2n+1=2.3+1=7. Here 2n>2n+1 holds.

Let us assume, m is a natural number and m>3. Let, Pm:2m>2m+1 is true. We have to prove that, Pm+1:2m+1>2(m+1)+1 is true also.

We know that m>1 2m>2(1)

Again, for m>3,2m>2m+1(2)

Combining (1) and (2) we get that ,

2m+2m>2+2m+1 2.2m>2m+32m+1>2(m+1)+1

Therefore, Pm+1 is true when Pm is true and m>3. So, following principle of mathematical induction Pn is true i.e., the inequality holds while n3.

Hence, 2n>2n+1 inequality holds while nN,n3.

Try Yourself

  1. Prove that,(2n)!22n(n!)21(3n+1)1/2nN

  2. nNwheren>11n+1+1n+2++12n>1324

  3. For n>1 prove that (2nn)>4nn+1

  4. If n2 then show that 1+14+19++1n2<21n

  5. If n is an integer and n3 then show that 2n+1<2n

  6. For n4 prove that n2<n!

Divisibility Problem

  1. Ifnis a even positive integer then prove that,xnynis divisible byx+y.

Solution: Let us assume,n=2m ;m=1,2,3,4

Pm:x2my2mwill be divisible byx+ywherem is a positive integer.(We have to prove)

Form=1, x2my2m=x2y2=(x+y)(xy) is divisible by(x+y). Therefore,P1is true.

Let us assumetis a positive integer andPtis true i.e.,x2ty2tis divisible by(x+y).

Now, we have to provePt+1is true i.e.,x2(t+1)y2(t+1)is divisible by(x+y).

x2(t+1)y2(t+1)=x2t+2y2t+2=x2t.x2y2t.y2=x2t.x2x2t.y2+x2ty2y2t.y2 =x2t(x2y2)+y2(x2ty2t)=x2t(x+y)(xy)+y2(x2ty2t)

x2t(x+y)(xy)is divisible by(x+y). Again, according to our assumptionx2ty2t is divisible

by(x+y).

 y2(x2ty2t)is divisible by(x+y)

x2t(x+y)(xy)+y2(x2ty2t)i.e.,x2(t+1)y2(t+1) is divisible by(x+y).Hence,Pt+1is true.

Ptis truePt+1 is also true.

Hence, by mathematical induction Pm is true where m is positive integer.

x2my2m is divisible by (x+y) i.e., xnynis divisible byx+y forn even positive integer. (Proved)

  1. Prove that p2+p+1|pn+1+(p+1)2n1nN

    Solution: Pn:p2+p+1|pn+1+(p+1)2n1

 n=1,pn+1+(p+1)2n1=p2+(p+1)2.11=p2+p+1, divisible by p2+p+1

P1 is true.

Let us assume, m is a natural number and Pm is true.  p2+p+1|pm+1+(p+1)2m1

Let us assume, pm+1+(p+1)2m1=k(p2+p+1) kZ+

Now, p(m+1)+1+(p+1)2(m+1)1=pm+2+(p+1)m+2=pm+2+(p+1)2m1.(p+1)2

 p(m+1)+1+(p+1)2(m+1)1=pm+2+(p+1)2m1.(p+1)2

=pm+2+[k(p2+p+1)pm+1](p+1)2

=pm+2pm+1(p+1)2+k(p2+p+1)(p+1)2

=pm+1(pp22p1)+k(p2+p+1)(p+1)2

=pm+1(p2+p+1)+k(p2+p+1)(p+1)2=p2+p+1{k(p+1)2pm+1}

Hence, p(m+1)+1+(p+1)2(m+1)1=p2+p+1{k(p+1)2pm+1} which is divisible byp2+p+1

Pm+1 is true

 Pm is truePm+1is true also.

Therefore, mathematical induction principle proves that p2+p+1|pn+1+(p+1)2n1nN

  1. Prove that 25|72n+(23n3).3n1nN

    Solution: Pn:25|72n+(23n3).3n1

n=1,72.1+23.13.311=50,divisble by25.P1is true.

Now, for Pm:25|72m+(23m3).3m1,mZ+()

Pm+1:72(m+1)+(23(m+1)3).3(m+1)1=72m.72+23m.3m

=72m.72+23m3.23.3m1.3=49.72m+23m3.3m1.24

=49.72m+49.23m3.3m125.23m3.3m1

=49(72m+23m3.3m1)25.23m3.3m1

Now, from () we get that 72m+(23m3).3m1 is divisible by 25

72m+(23m3).3m1=25k, wherek is positive integer.

=49.25k25.23m3.3m1=25(49k23m3.3m1)

Therefore it is divisible by 25. Hence, Pm+1 is true.

 Pm is truePm+1is true also.

By applying mathematical induction Pn is true for all n positive integer.

Therefore, 25|72n+(23n3).3n1 nN (Proved)

Try Yourself

  1. Prove that 576|52n+224n25nN

  2. Ifnis a even positive integer then prove that,anbnis divisible bya+b.

  3. Use the principle of mathematical induction and prove that

    i) 3|22n1,n1, ii) 7|32n+1+2n+2,n1 nZ+

  4. Using mathematical induction prove that 9|10n+3.4n+2+5nN

  5. Prove that for n0,nZ 17|3.52n+1+23n+1

Problem

  • Using principle of mathematical induction prove that 3|22n1 where nNwhere n1

Solution: Pn:3|22n1 Therefore, P1:22.11=41=3always divisible by 3P1is true

Let us consider Pm is true i.e., if n=m then Pn is true. Therefore, let us consider,

Pm:3|22m1nN;n1

22(m+1)1=22m+21=22m.221

=4.22m1=3.22m+(22m1)

As, 22m1always divisible by 3. Therefore it is clear that 22(m+1)1always divisible by 3

Pm+1:22(m+1)1always divisible by 3Pm+1is true.

As, P1 is true and Pm is true therefore Pm+1 is also true. Therefore, for n1 by induction it is clear that Pn is true.

3|22n1wherenNwhere n1(Proved)

Try Yourself

  1. Prove that for nN 17|32n+1+2n+2

  2. Prove that 6|n(n+1)(2n+1) for nN.

Problem

  1. Using mathematical induction prove that

    4+44+444+nno. of terms=481(10n+19n10)

Solution: Pn:4+44+444+nno. of terms=481(10n+19n10)

Therefore, P1:481(1029.110)=4 which is true. Therefore, P1 is true.

Pm:4+44+444+mno. of terms=481(10m+19n10) is true.

Now,

Pm+1:4+44+444+(m+1)no. of terms

=4+44+444+mno. of terms+444[where 4 is there (m+1) no. of times]

=481(10m+19m10)+444(m+1)no. of times()

444(m+1) no.of times=4+4.10+4.102+(m+1) no.of times=4.10m+11101=49(10m+11)

Form (*) we get that,

4+44+444+(m+1)no. of terms=481(10m+19m10)+49(10m+11)

=481(10m+19m10+9.10m+19)

=481[10.10m+19(m+1)10]

=481[10m+29(m+1)10]

Therefore, we can see that Pm+1 statement is true. So, we can see that P1 is true. As, Pm statement is true then Pm+1 is true. Hence, by principle of mathematical induction we can say that Pn is true. Therefore,

4+44+444+nno. of terms=481(10n+19n10)(Proved)

Try Yourself

  1. Using mathematical induction prove that

    6+66+666+6666+nno. of terms=227(10n+19n10)

  2. Using principle of mathematical induction show that

    .7+.77+.777+n no. of terms=7n9781(1110n)

Conclusion

The Induction Principle is of great importance in all of the fields of mathematics. This content is all about those inquisitive students who are trying to understand the basic & fundamental concepts of higher mathematics but are unable to grasp the insight behind induction principle.

In my previous Mathematical Induction-I I have tried to deliver a coherent understanding and a philosophical overview of Principle of Mathematical Induction. Here, I’ve discussed the mathematics behind principle of mathematical induction and some school level problem which we often deal with.

In a single content, it is not possible to cover all the necessary problem-solving strategies which is also required further. That’s why I’m willing to discuss lots more problems and problem-solving strategies on mathematical induction in my upcoming content which will be an asset to deal with critical problems from INMO, IMO, PUTNUM.

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