proof of the irrationality of root 2

Irrational numbers are those numbers which can not be expressed as a ratio between two integers. The first irrational number to be confronted by the Greeks was the square root of 2. Irrational numbers are still regarded as real numbers but they are not commensurate to any quantity. So they are in-commensurate numbers. Greek found that if the length of square figure is one then its diagonal is the square root of 2. They were puzzled by this weird number and its properties.

The diagonal of this square can not be multiple of any integer. On the other hand you can see that on a number line square root 2 can be represented as the distance which is the same as the diagonal of the square. All this was very troublesome for mathematics and mathematician in the ancient time. However we will proceed to the proof of the irrationality of the square root of two (2) :

Suppose root 2 is equal to the ratio of m to n. So

root 2 = m/n , if we square it we get 2n^2 = m^2 . That means that m^2 is an even number. Now let m = 2p so m^2 = 4p^2 . Thus we have 4p^2 = 2n^2 where p is the half of m. Hence 2p^2 = n^2 , and therefore n/p will also be equal to square root of 2. But we can repeat the argument : if n = 2q, p/q will also be the square root of 2 , and so on , through an unending series of numbers each of which is half of its predecessors. But this is clearly impossible, if we divide a number continuously by two 2 we must reach an odd number after a finite number of steps. Or we may put the argument in this way by assuming that the m/n we start with is in its lowest terms; in that case m and n can not both be even as we saw they were supposed to be.

So we must conclude that square root of 2 can not be represented as a ratio of two numbers. If we do we come to a contradiction as we have shown.

square root of two(2) can be expressed using circular and hyperbolic angle too. Here is an example .

circular and hyperbolic angle u

Where the symbol ^ is the exponential.

Real numbers and irrational number:

square root 2 as the diagonal of an square

As stated, no fraction will express exactly the length of the diagonal of a square whose side is one inch long. This problem was purely geometrical but the same problem arose as regards the solution of equations, though here it took a wider form, since it involved complex numbers.

It is clear that fractions can be found which approach nearer and nearer to having their square equal to 2. We can form an ascending series of fractions all of which have their squares less than 2, but differing from 2 in their later members by less than any assigned amount. This is to say, suppose I assign some small amount in advance, say one-billionth, it will be found that all the terms of our series after certain one, say, the tenth, have squares that differ from 2 by less than this amount. And if I had assigned a still smaller amount, it might have been necessary to go further along the series, but we should have reached sooner or later a term in the series, say, twentieth, after which all terms would have had squares differing from 2 by less than this still smaller amount. If we set to work to extract the square root of 2 by the usual arithmetical rule, we shall obtain an unending decimal which, taken to so-an-so many places, exactly fulfils the above conditions. We can equally form an descending series of fractions whose squares are all greater than 2, but greater by continually smaller amounts as we come to the later terms of the series, and differing , sooner or later, by less than any assigned amount. In this way we seem to be drawing a cordon around the square root of 2, and , it seems difficult to prove that, it can permanently escape us. But , it is not by this method that we will actually reach the square root of 2.


If we divide all ratios into two classes, according as their squares are less than 2 or not, we find that , among those whose squares are not less than 2 , all have their squares greater than 2. There is no maximum to the ratios whose square is less than 2, and no minimum to those whose square is greater than 2. There is no lower limit short of zero to the difference between the numbers whose square is little greater than 2. We can, in short, divide all ratios into two classes such that all the terms in one class are less than all in the other, there is no maximum to the one class, and there is no minimum to the other. Between these two classes , where root 2 ought to be there, there is actually nothing. We have not actually caught root 2. The cordon, though we have drawn as tight as possible , has been drawn in the wrong place and has not caught 2 .

The above method of dividing all the terms of a series into two classes, of which the one wholly precedes the other, was brought into prominence by Dedekind, and is therefore called a “Dedekind cut” . With respect to what happens at the point of section, there are four possibilities.

If we divide all ratios into two classes such that all of the terms of one class is greater than all the terms of the other then there may arise four case: 1) there may be a minimum to upper the upper section and a maximum to the lower section 2) there may be a maximum to the one and no minimum to the other 3) there may be no maximum to the one , but a minimum to the other 4) there may be neither a maximum to the one nor a minimum to the other. Of these four cases, the first can be illustrated by any series in which there are consecutive terms: in the series of integers, the lower class may contain a maximum number n and the upper class can contain a minimum n+1 . The second of these cases will be illustrated in the series of ratios if we take as our lower section all the ratios up to and including 1, and in our upper section all the ratios greater than 1. The third case is illustrated if we take for our lower section all ratios from 1 upward (including 1 itself) . The fourth case, as we have seen , is illustrated if we put in our lower section all the ratios whose square is less than 2 , and in our upper section all the ratios whose square is greater than 2.

We may neglect the first of our four cases, since it only arises in series where there are consecutive terms. In the second of our four cases, we say that the maximum of the lower section is the lower limit of the upper section, or of any set of terms chosen out of the upper section in such a way that no term of the upper section is the upper limit of the lower section, or of any set of terms chosen out of the lower section is after all of them. In the fourth case, we say that there is a “gap”: neither the upper section or lower section has a limit or last term. In this case, we may say that we have an irrational section, for the class of ratios which do not have limit corresponds to irrationals.

What delayed the true theory of irrational was a erroneous belief that there must be “limits” of series of ratios. The notion of “limit” is of the utmost importance , and before proceeding further it will be well to define it.

A term x is said to be an “upper limit” of a class s with respect to a relation P if (1) s has no maximum in P , (2) every member of s which belongs to the field of P precedes x , (3) every member of the field of P which precedes x precedes some member of s . ( by “precedes ” we mean “has the relation P to”).

This presupposes the following definition of “maximum” : –

A term x is said to be maximum of a class s with respect to a relation P if x is a member of s and of the field P and does not have the relation P to any other member of s.

These definitions do not demand that the terms to which they are applied should be quantitative. For example, given a series of moments of time arranged by earlier and later, their maximum (if any) will be the last of the moments.; but if they are arranged by later and earlier, their maximum (if any) will be the first of the moments.

The “minimum” of a class with respect to P is its maximum with respect to the converse of P; and the “lower limit” with respect to P is the “upper limit” with respect to the converse of P.

The notions of limit and maximum do not essentially demand that the relation in respect to which they are defined should be serial, but they have few important applications except to cases when the relation is serial and quasi-serial. A notion which is often important is the notion “upper limit or maximum” to which we may give the name “upper boundary”. Thus the “upper boundary ” of a set of terms chosen out of a series is their last member if they have one, but , if not it is the first term after all of them, if there is such a term. If there is neither a maximum nor a limit, there is no upper boundary. The “lower boundary” is the lower limit or minimum. Readers at this moment, should be aware that limit and maximum are not the same concept or the same thing.

Reverting to the four kinds of Dedekind section, we see that in the case of the first three kinds each section has a boundary( upper or lower as the case may be), while in the fourth kind neither has a boundary. It is also clear that, whenever the lower section has an upper boundary , the upper section has a lower boundary. In the second and third cases, the two boundaries are identical; in the first , they are consecutive terms of the series.

A series is called “Dedekindian” when every section has a boundary, upper or lower as the case may be.

We have seen that the series of ratios in order of magnitude is not Dedekindian.

From the habit of being influenced by spatial imagination, people have supposed that series must have limits in cases where it seems odd if they do not. Thus, perceiving that there was no rational limit to the ratios whose square is less than 2 , they allowed themselves to “postulate” an irrational limit, which was to fill the Dedekindian gap. Dedekind, in the above mentioned work, set up the axiom that the gap must always be filled, i.e that every section must have a boundary . It is for this reason that series where his axiom is verified are called “Dedekindian.” But there are an infinite number of series for which it is not verified.

It is clear that an irrational Dedekind cut in some way “represents” an irrational. But this is not true to say that an irrational must be the limit of a set of ratios. Just as ratios whose denominator is 1 is not identical with integers, so those rational numbers which can be greater or less than irrationals, or can have irrationals as their limits, must not be identified with ratios. We have to define a new kind of numbers called “real numbers” of which some will be rational and some irrationals. Those that are rationals correspond to ratios, in the same kind of way in which the ration n/1 corresponds to the integer n; but they are not the same as ratios. In order to decide what they are to be , let us observer that an irrational is represented by an irrational cut, and a cut is represented by its lower section. Let us confine ourselves to cuts in which the lower section has no maximum; in this case we call the lower section a “segment”. Then those segments that correspond to ratios are those that consist of all ratios less than a certain ratio that they correspond to, which is their boundary; while those that represent irrationals are those that have no boundary. Segments , both those that have boundaries and those that do not, are such that , of any two pertaining to one series one is the part of the other. In that case all those segments can be arranged by the relation of whole and part. A series in which there are Dedekind gaps, i.e in which there are segments that have no boundary, will give rise to more segments than its terms, since each term will define a segment having that term for boundary , and the segments without boundaries will be extra.

We are now in a position to define a real number and an irrational number.

A real number is a segment of the series of ratios in order of magnitude.

An ‘irrational number” is a segment of the series of ratios which has no boundary.

A “rational real number ” is a segment of the series of ratios which has a boundary.

Thus a rational real number consists of all ratios less than a certain ratio, and it is the rational real number corresponding to that ratio. The real number 1, for instance , is the class of proper fractions.

In this way we come up with a new theory of real numbers.

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