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A Logarex system Darmstadt slide rule with 7 and 6 on A and B scales, and square roots of 6 and of 7 on C and D scales, which can be read as slightly less than 2.45 and somewhat more than 2.64, respectively. The square root of 7 is the positive real number that, when multiplied by itself, gives the prime number 7.
Otherwise, take a to be the irrational number √ 2 √ 2 and b = √ 2. Then a b = (√ 2 √ 2) √ 2 = √ 2 √ 2 · √ 2 = √ 2 2 = 2, which is rational. Although the above argument does not decide between the two cases, the Gelfond–Schneider theorem shows that √ 2 √ 2 is transcendental, hence irrational.
Rational numbers have irrationality exponent 1, while (as a consequence of Dirichlet's approximation theorem) every irrational number has irrationality exponent at least 2. On the other hand, an application of Borel-Cantelli lemma shows that almost all numbers, including all algebraic irrational numbers , have an irrationality exponent exactly ...
If the ω(x, n) are finite but unbounded, x is called a T number. x is algebraic if and only if ω(x) = 0. Clearly the Liouville numbers are a subset of the U numbers. William LeVeque in 1953 constructed U numbers of any desired degree. [24] The Liouville numbers and hence the U numbers are uncountable sets. They are sets of measure 0.
The classic proof that the square root of 2 is irrational is a refutation by contradiction. [11] Indeed, we set out to prove the negation ¬ ∃ a, b ∈ . a/b = √ 2 by assuming that there exist natural numbers a and b whose ratio is the square root of two, and derive a contradiction.
This shows that any irrational number has irrationality measure at least 2. The Thue–Siegel–Roth theorem says that, for algebraic irrational numbers, the exponent of 2 in the corollary to Dirichlet’s approximation theorem is the best we can do: such numbers cannot be approximated by any exponent greater than 2.
In proof by contradiction, also known by the Latin phrase reductio ad absurdum (by reduction to the absurd), it is shown that if some statement is assumed true, a logical contradiction occurs, hence the statement must be false. A famous example involves the proof that is an irrational number:
Thus the group-theoretic notion of commensurability generalizes the concept for real numbers. There is a similar notion for two groups which are not given as subgroups of the same group. Two groups G 1 and G 2 are (abstractly) commensurable if there are subgroups H 1 ⊂ G 1 and H 2 ⊂ G 2 of finite index such that H 1 is isomorphic to H 2.