Transcendental number

Properties

A transcendental number is a (possibly complex) number that is not the root of any integer polynomial. Every real transcendental number must also be irrational, since every rational number is the root of some integer polynomial of degree one. The set of transcendental numbers is uncountably infinite. Since the polynomials with rational coefficients are countable, and since each such polynomial has a finite number of zeroes, the algebraic numbers must also be countable. However, Cantor's diagonal argument proves that the real numbers (and therefore also the complex numbers) are uncountable. Since the real numbers are the union of algebraic and transcendental numbers, it is impossible for both subsets to be countable. This makes the transcendental numbers uncountable.

No rational number is transcendental and all real transcendental numbers are irrational. The irrational numbers contain all the real transcendental numbers and a subset of the algebraic numbers, including the quadratic irrationals and other forms of algebraic irrationals.

Applying any non-constant single-variable algebraic function to a transcendental argument yields a transcendental value. For example, from knowing that π is transcendental, it can be immediately deduced that numbers such as ${\displaystyle 5\pi }$, ${\displaystyle \frac{\pi -3}{\sqrt{2}}}$, ${\displaystyle (\sqrt{\pi }-\sqrt{3}{)}^8}$, and ${\displaystyle \sqrt[4]{{\pi }^5+7}}$ are transcendental as well.

However, an algebraic function of several variables may yield an algebraic number when applied to transcendental numbers if these numbers are not algebraically independent. For example, π and (1 − π) are both transcendental, but π + (1 − π) = 1 is not. It is unknown whether e + π, for example, is transcendental, though at least one of e + π and eπ must be transcendental. More generally, for any two transcendental numbers a and b, at least one of a + b and ab must be transcendental. To see this, consider the polynomial (x − a)(x − b) = x² − (a + b) x + a b . If (a + b) and a b were both algebraic, then this would be a polynomial with algebraic coefficients. Because algebraic numbers form an algebraically closed field, this would imply that the roots of the polynomial, a and b, must be algebraic. But this is a contradiction, and thus it must be the case that at least one of the coefficients is transcendental.

The non-computable numbers are a strict subset of the transcendental numbers.

All Liouville numbers are transcendental, but not vice versa. Any Liouville number must have unbounded partial quotients in its simple continued fraction expansion. Using a counting argument one can show that there exist transcendental numbers which have bounded partial quotients and hence are not Liouville numbers.

Using the explicit continued fraction expansion of e, one can show that e is not a Liouville number (although the partial quotients in its continued fraction expansion are unbounded). Kurt Mahler showed in 1953 that π is also not a Liouville number. It is conjectured that all infinite continued fractions with bounded terms, that have a "simple" structure, and that are not eventually periodic are transcendental (in other words, algebraic irrational roots of at least third degree polynomials do not have apparent pattern in their continued fraction expansions, since eventually periodic continued fractions correspond to quadratic irrationals, see Hermite's problem).

Numbers proven to be transcendental

Numbers proven to be transcendental:

• π (by the Lindemann–Weierstrass theorem).
• ${\displaystyle e^a}$ if a is algebraic and nonzero (by the Lindemann–Weierstrass theorem), in particular Euler's number e.
• ${\displaystyle e^{\pi \sqrt{n}}}$ where n is a positive integer; in particular Gelfond's constant ${\displaystyle e^{\pi }}$ (by the Gelfond–Schneider theorem).
• Algebraic combinations of π and ${\displaystyle e^{\pi \sqrt{n}},n\in {\mathbb{Z}}^{+}}$ such as ${\displaystyle \pi +e^{\pi }}$ and ${\displaystyle \pi e^{\pi }}$ (following from their algebraic independence).
• ${\displaystyle a^b}$ where a is algebraic but not 0 or 1, and b is irrational algebraic, in particular the Gelfond–Schneider constant ${\displaystyle 2^{\sqrt{2}}}$ (by the Gelfond–Schneider theorem).
• The natural logarithm ln(a) if a is algebraic and not equal to 0 or 1, for any branch of the logarithm function (by the Lindemann–Weierstrass theorem).
• ${\displaystyle {\log }_b(a)}$ if a and b are positive integers not both powers of the same integer, and a is not equal to 1 (by the Gelfond–Schneider theorem).
• All numbers of the form ${\displaystyle \pi +{\beta }_1\ln (a_1)+\cdots +{\beta }_n\ln (a_n)}$ are transcendental, where ${\displaystyle {\beta }_j}$ are algebraic for all ${\displaystyle 1\le j\le n}$ and ${\displaystyle a_j}$ are non-zero algebraic for all ${\displaystyle 1\le j\le n}$ (by Baker's theorem).
• The trigonometric functions sin(x), cos(x) and their hyperbolic counterparts, for any nonzero algebraic number x, expressed in radians (by the Lindemann–Weierstrass theorem).
• Non-zero results of the inverse trigonometric functions arcsin(x), arccos(x) and their hyperbolic counterparts, for any algebraic number x (by the Lindemann–Weierstrass theorem).
• ${\displaystyle {\pi }^{-1}\arctan (x)}$, for rational x such that ${\displaystyle x\notin \{0,\pm 1\}}$.
• The Dottie number d (the fixed point of the cosine function) – the unique real solution to the equation ${\displaystyle \cos (x)=x}$, where x is in radians (by the Lindemann–Weierstrass theorem).
• ${\displaystyle W(a)}$ if a is algebraic and nonzero, for any branch of the Lambert W function (by the Lindemann–Weierstrass theorem), in particular the omega constant Ω.
• ${\displaystyle W(r,a)}$ if both a and the order r are algebraic such that ${\displaystyle a\ne 0}$, for any branch of the generalized Lambert W function.
• ${\displaystyle {\sqrt{x}}_s}$, the square super-root of any natural number is either an integer or transcendental (by the Gelfond–Schneider theorem).
• Values of the gamma function of rational numbers that are of the form ${\displaystyle \mathrm{\Gamma }(n/2),\mathrm{\Gamma }(n/3),\mathrm{\Gamma }(n/4)}$ or ${\displaystyle \mathrm{\Gamma }(n/6)}$.
• Algebraic combinations of π and ${\displaystyle \mathrm{\Gamma }(1/3)}$ or of π and ${\displaystyle \mathrm{\Gamma }(1/4)}$ such as the lemniscate constant ${\displaystyle \varpi }$ (following from their respective algebraic independences).
• The values of Beta function ${\displaystyle \mathrm{B}(a,b)}$ if ${\displaystyle a,b}$ and ${\displaystyle a+b}$ are non-integer rational numbers.
• The Bessel function of the first kind ${\displaystyle J_{\nu }(x)}$, its first derivative, and the quotient ${\displaystyle \frac{J_{\nu }^{\prime }(x)}{J_{\nu }(x)}}$ are transcendental when ν is rational and x is algebraic and nonzero, and all nonzero roots of ${\displaystyle J_{\nu }(x)}$ and ${\displaystyle J_{\nu }^{\prime }(x)}$ are transcendental when ν is rational.
• The number ${\displaystyle \frac{\pi }{2}\frac{Y_0(2)}{J_0(2)}-\gamma }$, where ${\displaystyle Y_{\alpha }(x)}$ and ${\displaystyle J_{\alpha }(x)}$ are Bessel functions and γ is the Euler–Mascheroni constant.
• Values of the Fibonacci zeta function at positive even arguments.
• Any Liouville number, in particular: Liouville's constant ${\displaystyle \sum _{k=1}^{\mathrm{\infty }}\frac{1}{{10}^{k!}}}$.
• Numbers with irrationality measure larger than 2, such as the Champernowne constant ${\displaystyle C_{10}}$ and Cahen's constant (by Roth's theorem).
• Numbers artificially constructed not to be algebraic periods.
• Any non-computable number, in particular: Chaitin's constant.
• Constructed irrational numbers which are not simply normal in any base.
• Any number for which the digits with respect to some fixed base form a Sturmian word.
• Any irrational number for which the nth digit can be generated by a finite-state machine with b states from the base-b expansion of n.
• Any number for which the simple continued fraction is aperiodic (i.e. a non-quadratic irrational number) and the nth term can be generated by a finite-state machine with b states from the base-b expansion of n.
• The Prouhet–Thue–Morse constant and the related rabbit constant.
• The Komornik–Loreti constant.
• The paperfolding constant (also named as "Gaussian Liouville number").
• The values of the infinite series with fast convergence rate as defined by Y. Gao and J. Gao, such as ${\displaystyle \sum _{n=1}^{\mathrm{\infty }}\frac{3^n}{2^{3^n}}}$.
• Any number of the form ${\displaystyle \sum _{n=0}^{\mathrm{\infty }}\frac{E_n({\beta }^{r^n})}{F_n({\beta }^{r^n})}}$ (where ${\displaystyle E_n(z)}$, ${\displaystyle F_n(z)}$ are polynomials in variables ${\displaystyle n}$ and ${\displaystyle z}$, ${\displaystyle \beta }$ is algebraic and ${\displaystyle \beta \ne 0}$, ${\displaystyle r}$ is any integer greater than 1).
• Numbers of the form ${\displaystyle \sum _{k=0}^{\mathrm{\infty }}{10}^{-b^k}}$ and ${\displaystyle \sum _{k=0}^{\mathrm{\infty }}{10}^{-\lfloor b^k\rfloor }}$ For b > 1 where ${\displaystyle b\mapsto \lfloor b\rfloor }$ is the floor function.
• The numbers ${\displaystyle \alpha =\mathrm{3.3003300000...}}$ and ${\displaystyle {\alpha }^{-1}=\mathrm{0.3030000030...}}$ with only two different decimal digits whose nonzero digit positions are given by the Moser–de Bruijn sequence and its double.
• The values of the Rogers-Ramanujan continued fraction ${\displaystyle R(q)}$ where ${\displaystyle q\in \mathbb{C}}$ is algebraic and ${\displaystyle 0<|q|<1}$. The lemniscatic values of theta function ${\displaystyle \sum _{n=-\mathrm{\infty }}^{\mathrm{\infty }}{q}^{n^2}}$ (under the same conditions for ${\displaystyle q}$) are also transcendental.
• j(q) where ${\displaystyle q\in \mathbb{C}}$ is algebraic but not imaginary quadratic (i.e, the exceptional set of this function is the number field whose degree of extension over ${\displaystyle \mathbb{Q}}$ is 2).
• The constants ${\displaystyle {ϵ}_k}$ and ${\displaystyle {\nu }_k}$ in the formula for first index of occurrence of Gijswijt's sequence, where k is any integer greater than 1.
• Any hypertranscendental number, a number that is not the value at an algebraic point of a function which is the solution of an algebraic differential equation with coefficients in ${\displaystyle \mathbb{Z}[r]}$ and with algebraic initial conditions.

Conjectured transcendental numbers

Numbers which have yet to be proven to be either transcendental or algebraic:

• Most nontrivial combinations of two or more transcendental numbers are themselves not known to be transcendental or even irrational: eπ, e + π, π^π, e^e, π^e, π^√2, e^π2. It has been shown that both e + π and π/e do not satisfy any polynomial equation of degree ≤ 8 and integer coefficients of average size 10⁹. At least one of the numbers e^e and e^e2 is transcendental. Since the field of algebraic numbers is algebraically closed and e and π are roots of the polynomial x² − (e + π)x + eπ, at least one of the numbers eπ and e + π is transcendental. Schanuel's conjecture would imply that all of the above numbers are transcendental and algebraically independent.
• The Euler–Mascheroni constant γ: In 2010 it has been shown that an infinite list of Euler-Lehmer constants (which includes γ/4) contains at most one algebraic number. In 2012 it was shown that at least one of γ and the Gompertz constant δ is transcendental.
• The values of the Riemann zeta function ζ(n) at odd positive integers ${\displaystyle n\ge 3}$; in particular Apéry's constant ζ(3), which is known to be irrational. For the other numbers ζ(5), ζ(7), ζ(9), ... even this is not known. For any non-negative integer k, at least one of the numbers ζ(4k+3) and ${\displaystyle \sum _{n=1}^{\mathrm{\infty }}\frac{1}{n^{4k+3}(e^{2\pi n}-1)}}$ is transcendental.
• The values of the Dirichlet beta function β(n) at even positive integers ${\displaystyle n\ge 2}$; in particular Catalan's Constant β(2). (None of them are known to be irrational.)
• The Mills' constant (which is known to be irrational).
• Values of the Gamma Function Γ(1/n) for positive integers ${\displaystyle n=5}$ and ${\displaystyle n\ge 7}$ are not known to be irrational, let alone transcendental. For ${\displaystyle n\ge 2}$ at least one the numbers Γ(1/n) and Γ(2/n) is transcendental.
• Any number given by some kind of limit that is not obviously algebraic.

Proofs for specific numbers

A proof that e is transcendental

The first proof that the base of the natural logarithms, e, is transcendental dates from 1873. We will now follow the strategy of David Hilbert (1862–1943) who gave a simplification of the original proof of Charles Hermite. The idea is the following:

Assume, for purpose of finding a contradiction, that e is algebraic. Then there exists a finite set of integer coefficients c₀, c₁, ..., c_n satisfying the equation: $${\displaystyle c_0+c_{1}e+c_2{e}^2+\cdots +c_n{e}^n=0,c_0,c_n\ne 0.}$$ It is difficult to make use of the integer status of these coefficients when multiplied by a power of the irrational e, but we can absorb those powers into an integral which “mostly” will assume integer values. For a positive integer k, define the polynomial $${\displaystyle f_k(x)=x^k{\left[(x-1)\cdots (x-n)\right]}^{k+1},}$$ and multiply both sides of the above equation by $${\displaystyle {\int }_0^{\mathrm{\infty }}{f}_k(x)e^{-x}\mathrm{d}x,}$$ to arrive at the equation: $${\displaystyle c_0\left({\int }_0^{\mathrm{\infty }}{f}_k(x)e^{-x}\mathrm{d}x\right)+c_{1}e\left({\int }_0^{\mathrm{\infty }}{f}_k(x)e^{-x}\mathrm{d}x\right)+\cdots +c_n{e}^n\left({\int }_0^{\mathrm{\infty }}{f}_k(x)e^{-x}\mathrm{d}x\right)=0.}$$

By splitting respective domains of integration, this equation can be written in the form $${\displaystyle P+Q=0}$$ where $${\displaystyle \begin{array}{cc}P& =c_0\left({\int }_0^{\mathrm{\infty }}{f}_k(x)e^{-x}\mathrm{d}x\right)+c_{1}e\left({\int }_1^{\mathrm{\infty }}{f}_k(x)e^{-x}\mathrm{d}x\right)+c_2{e}^2\left({\int }_2^{\mathrm{\infty }}{f}_k(x)e^{-x}\mathrm{d}x\right)+\cdots +c_n{e}^n\left({\int }_n^{\mathrm{\infty }}{f}_k(x)e^{-x}\mathrm{d}x\right)\\ Q& =c_{1}e\left({\int }_0^1{f}_k(x)e^{-x}\mathrm{d}x\right)+c_2{e}^2\left({\int }_0^2{f}_k(x)e^{-x}\mathrm{d}x\right)+\cdots +c_n{e}^n\left({\int }_0^n{f}_k(x)e^{-x}\mathrm{d}x\right)\end{array}}$$ Here P will turn out to be an integer, but more importantly it grows quickly with k.

Lemma 1

There are arbitrarily large k such that ${\displaystyle \frac{P}{k!}}$ is a non-zero integer.

Proof. Recall the standard integral (case of the Gamma function) $${\displaystyle {\int }_0^{\mathrm{\infty }}{t}^j{e}^{-t}\mathrm{d}t=j!}$$ valid for any natural number ${\displaystyle j}$. More generally,

if ${\displaystyle g(t)=\sum _{j=0}^m{b}_j{t}^j}$ then ${\displaystyle {\int }_0^{\mathrm{\infty }}g(t)e^{-t}\mathrm{d}t=\sum _{j=0}^m{b}_{j}j!}$.

This would allow us to compute ${\displaystyle P}$ exactly, because any term of ${\displaystyle P}$ can be rewritten as $${\displaystyle c_a{e}^a{\int }_a^{\mathrm{\infty }}{f}_k(x)e^{-x}\mathrm{d}x=c_a{\int }_a^{\mathrm{\infty }}{f}_k(x)e^{-(x-a)}\mathrm{d}x=\left\{\begin{array}{cc}t& =x-a\\ x& =t+a\\ \mathrm{d}x& =\mathrm{d}t\end{array}\right\}=c_a{\int }_0^{\mathrm{\infty }}{f}_k(t+a)e^{-t}\mathrm{d}t}$$ through a change of variables. Hence $${\displaystyle P=\sum _{a=0}^n{c}_a{\int }_0^{\mathrm{\infty }}{f}_k(t+a)e^{-t}\mathrm{d}t={\int }_0^{\mathrm{\infty }}(\sum _{a=0}^n{c}_a{f}_k(t+a))e^{-t}\mathrm{d}t}$$ That latter sum is a polynomial in ${\displaystyle t}$ with integer coefficients, i.e., it is a linear combination of powers ${\displaystyle t^j}$ with integer coefficients. Hence the number ${\displaystyle P}$ is a linear combination (with those same integer coefficients) of factorials ${\displaystyle j!}$; in particular ${\displaystyle P}$ is an integer.

Smaller factorials divide larger factorials, so the smallest ${\displaystyle j!}$ occurring in that linear combination will also divide the whole of ${\displaystyle P}$. We get that ${\displaystyle j!}$ from the lowest power ${\displaystyle t^j}$ term appearing with a nonzero coefficient in ${\displaystyle \sum _{a=0}^n{c}_a{f}_k(t+a)}$, but this smallest exponent ${\displaystyle j}$ is also the multiplicity of ${\displaystyle t=0}$ as a root of this polynomial. ${\displaystyle f_k(x)}$ is chosen to have multiplicity ${\displaystyle k}$ of the root ${\displaystyle x=0}$ and multiplicity ${\displaystyle k+1}$ of the roots ${\displaystyle x=a}$ for ${\displaystyle a=1,\dots ,n}$, so that smallest exponent is ${\displaystyle t^k}$ for ${\displaystyle f_k(t)}$ and ${\displaystyle t^{k+1}}$ for ${\displaystyle f_k(t+a)}$ with ${\displaystyle a>0}$. Therefore ${\displaystyle k!}$ divides ${\displaystyle P}$.

To establish the last claim in the lemma, that ${\displaystyle P}$ is nonzero, it is sufficient to prove that ${\displaystyle k+1}$ does not divide ${\displaystyle P}$. To that end, let ${\displaystyle k+1}$ be any prime larger than ${\displaystyle n}$ and ${\displaystyle |c_0|}$. We know from the above that ${\displaystyle (k+1)!}$ divides each of ${\displaystyle c_a{\int }_0^{\mathrm{\infty }}{f}_k(t+a)e^{-t}\mathrm{d}t}$ for ${\displaystyle 1⩽a⩽n}$, so in particular all of those are divisible by ${\displaystyle k+1}$. It comes down to the first term ${\displaystyle c_0{\int }_0^{\mathrm{\infty }}{f}_k(t)e^{-t}\mathrm{d}t}$. We have (see falling and rising factorials) $${\displaystyle f_k(t)=t^k[(t-1)\cdots (t-n){]}^{k+1}=[(-1{)}^n(n!){]}^{k+1}{t}^k+\text{higher degree terms}}$$ and those higher degree terms all give rise to factorials ${\displaystyle (k+1)!}$ or larger. Hence $${\displaystyle P\equiv c_0{\int }_0^{\mathrm{\infty }}{f}_k(t)e^{-t}\mathrm{d}t\equiv c_0[(-1{)}^n(n!){]}^{k+1}k!(\mathrm{mod}(k+1))}$$ That right hand side is a product of nonzero integer factors less than the prime ${\displaystyle k+1}$, therefore that product is not divisible by ${\displaystyle k+1}$, and the same holds for ${\displaystyle P}$; in particular ${\displaystyle P}$ cannot be zero.

Lemma 2

For sufficiently large k, ${\displaystyle \left|\frac{Q}{k!}\right|<1}$.

Proof. Note that

$${\displaystyle \begin{array}{cc}{f}_k{e}^{-x}& =x^k{\left[(x-1)(x-2)\cdots (x-n)\right]}^{k+1}{e}^{-x}\\ & ={\left(x(x-1)\cdots (x-n)\right)}^k\cdot \left((x-1)\cdots (x-n)e^{-x}\right)\\ & =u(x{)}^k\cdot v(x)\end{array}}$$

where u(x), v(x) are continuous functions of x for all x, so are bounded on the interval [0, n]. That is, there are constants G, H > 0 such that

$${\displaystyle \left|f_k{e}^{-x}\right|\le |u(x){|}^k\cdot |v(x)|<G^{k}H\text{ for }0\le x\le n.}$$

So each of those integrals composing Q is bounded, the worst case being

$${\displaystyle \left|{\int }_0^n{f}_k{e}^{-x}\mathrm{d}x\right|\le {\int }_0^n\left|f_k{e}^{-x}\right|\mathrm{d}x\le {\int }_0^n{G}^{k}H\mathrm{d}x=n{G}^{k}H.}$$

It is now possible to bound the sum Q as well:

$${\displaystyle |Q|<G^k\cdot nH\left(|c_1|e+|c_2|e^2+\cdots +|c_n|e^n\right)=G^k\cdot M,}$$

where M is a constant not depending on k. It follows that

$${\displaystyle \left|\frac{Q}{k!}\right|<M\cdot \frac{G^k}{k!}\to 0\text{ as }k\to \mathrm{\infty },}$$

finishing the proof of this lemma.

Conclusion

Choosing a value of k that satisfies both lemmas leads to a non-zero integer ${\displaystyle \left(\frac{P}{k!}\right)}$ added to a vanishingly small quantity ${\displaystyle \left(\frac{Q}{k!}\right)}$ being equal to zero: an impossibility. It follows that the original assumption, that e can satisfy a polynomial equation with integer coefficients, is also impossible; that is, e is transcendental.

The transcendence of π

A similar strategy, different from Lindemann's original approach, can be used to show that the number π is transcendental. Besides the gamma-function and some estimates as in the proof for e, facts about symmetric polynomials play a vital role in the proof.

For detailed information concerning the proofs of the transcendence of π and e, see the references and external links.