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# Number Theory/irrational numbers

Question
Hello Sir,

I am knew to this site and you can help me out of the following two questions.

1) Prove that the number 0.1010010001…1000…01…  is irrational
number.

2)Prove that for every positive rational number r satisfying the
condition r^2 < 2 one can always find a larger rational number
r + h (h > 0) for which (r +h) (r + h) < 2.

High regards and thanks in advance.
Vinod verma

Hello Vinod

(1) Your number is a decimal which is neither terminating nor recurring.  Therefore it is irrational.  What is more difficult to prove is that is non-algebraic.  That is,it is not the root of a polynomial equation with integer coefficients.  Liouville proved that, and you can find proofs on the internet, but they are too complex for me to type here.

(2) The infinite continued fraction for root 2 is [1,2,2,2,2...]
Its first convergents are 1/1, 1 + 1/2 = 3/2, 1+1/(2+1/2)=7/5, 1+1/(2+1/(2+1/2))=17/12.
ie. 1/1, 3/2, 7/5, 17/12.  The numerator and denominator of each fraction except the first two can be obtained by a simple rule. Double the previous and add the penultimate.  For example 17=2*7+3 and 12=2*5+2.  All the convergents in odd positions are less than root2, the others greater.  As the convergents get closer and closer to root2, you can find one better than r.

I also tried a do-it yourself method.  2-r^2 is a small number, so x = (2-r^2)^2 is very small.
Then r+x is a better approximation.  I've tested it and it certainly works, but I've not been able to prove it yet.

Regards

vijilant
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Number Theory

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#### Vijilant

##### Expertise

Most questions on number theory, divisibility, primes, Euclidean algorithm, Fermat`s theorem, Wilson`s theorem, factorisation, euclidean algorithm, diophantine equations, Chinese remainder theorem, group theory, congruences, continued fractions.

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Teacher of math for 53 years

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Journal of mathematics and its applications M500 magazine

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State Scholarship 1955 Highest Score in Yorkshire on OU course MST209 £50 prize First class honours in OU BA Mathematics

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I taught John Birt, former Director of the BBC in 1961. His homework book was the most perfect I have ever marked. And also the most neat. I could tell he was destined for great things. One of my classmates was the poet Roger McGough, and I have a mention in his autobiography.