Irrational Numbers from Adjacent Natural Numbers (Part IVa)

An Inductive Proof and the Sequence of Irrational Numbers

It has been shown (Between_two_Rational_Numbers on Wiki) that between two real numbers (ℝ) there exists an irrational number. Accordingly, it will be shown here that between two adjacent natural numbers ( ) there exists an irrational number (ℝ\ℚ). In addition, an arithmetic progression or sequence of irrational numbers will be generated where the initial common difference (d) between two irrational numbers generated from [(x + y)y] ∕ 2 is an irrational number between 3 < d < 7.5 which approaches d=1 as the number of natural numbers grows without bound. Therefore, we can proceed as follows:

If x and y are consecutive natural numbers where y = x + 1 then we can show by induction that

x < [(x + y)y] ∕ 2 < y   (1)

where x,y ∈ ℕ and [(x + y)y] ∕ 2 is an irrational number.

First substituting x + 1 for y, affords

x < ((2x + 1)(x + 1)) ∕ 2 < x + 1   (2)

then substituting x = 1 in the basis step, affords:

1 < 3 < 2 which is true.

Squaring the left, center and right parts of inequality (2)

x2 < (2x2 + 3x + 1) ∕ 2 < (x + 1)2  (3)


To prove the chain notation for all numbers we add the equation (2x + 1) to all three parts of the inequality as follows:


x2 + 2x + 1 < (2x2 + 3x + 1) ∕ 2 + 2x + 1 < (x+1)2 + 2x + 1   (4)

Multiplying 2x + 1 by 2 ∕ 2, expanding and adding terms

x2 + 2x + 1 < (2x2 + 7x + 3) ∕ 2 < x2 + 4x + 2     (5)

since

(2x2 + 7x + 3) ∕ 2 < (2x2 + 7x + 6) ∕ 2 and

x2 + 4x + 2 < x2 + 4x + 4 then

x2 + 2x + 1 < (2x2 + 7x + 3) ∕ 2 < (2x2 + 7x + 6) ∕ 2 < x2 + 4x + 2 < x2 + 4x + 4  (6)

which can be reduced to

x2 + 2x + 1 < (2x2 + 7x + 6) ∕ 2 < x2 + 4x + 4   (7)

multiplying the inequality by 2

2x2 + 4x + 2 < 2x2 + 7x + 6 < 2x2 + 8x + 8    (8)

which is true for all x

Factoring of (7), affords

(x + 1)2 < ½(x + 2)(2x + 3) < (x + 2)2    (9)

Taking the square roots of all the terms

[½(2x + 3)(x + 2)]     (10)

and rearranging to

½[(x + 1) + (x + 2)](x + 2)  (11)

Substituting x = y − 1 for the last two (x + 2) terms, finally affords

(x + 1) < ½[(x + 1) + (y + 1)](y + 1) < (y + 1)  (12)

and proves the inequality x < ½[(x + y)y] < y for all x and for y = x + 1.

The sequence generated from [(x + y)y] ∕ 2, is shown in Table I where the values of x ranging from 1 to 10 and those of x+1 = y from 2 to 11 are multiplied. The value for each [(x + y)y] ∕ 2, to six decimal places, is irrational and each of these values indeed falls between two consecutive natural numbers. In addition, two larger values for x and x+1 are shown in the last row with a d value even closer to 1. The third column heading SQRT is equal to ((x+y)(y)) ∕ 2 which in turn is equal to ((2x+1)(x+1)) ∕ 2.

Table I Irrational (Sequence)
x x+1 SQRTd
121.732051
1.006561
232.738613
1.003044
343.741657
1.001759
454.743416
1.001147
565.744563
1.000806
676.745359
1.000598
787.745967
1.000461
898.746428
1.000366
9109.746794
1.000299
101110.747093
9910099.749687
1.000003
100101100.749690

Part IVb is the second part where the equation [(x + y)x] ∕ 2 has x instead of y as a variable.

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