# Six exponentials theorem

In mathematics, specifically transcendental number theory, the **six exponentials theorem** is a result that, given the right conditions on the exponents, guarantees the transcendence of at least one of a set of exponentials.

## Statement

If *x*_{1}, *x*_{2},..., *x*_{d} are *d* complex numbers that are linearly independent over the rational numbers, and *y*_{1}, *y*_{2},...,*y*_{l} are *l* complex numbers that are also linearly independent over the rational numbers, and if *dl* > *d* + *l*, then at least one of the following *dl* numbers is transcendental:

The most interesting case is when *d* = 3 and *l* = 2, in which case there are six exponentials, hence the name of the result. The theorem is weaker than the related but thus far unproved four exponentials conjecture, whereby the strict inequality *dl* > *d* + *l* is replaced with *dl* ≥ *d* + *l*, thus allowing *d* = *l* = 2.

The theorem can be stated in terms of logarithms by introducing the set *L* of logarithms of algebraic numbers:

The theorem then says that if λ_{ij} are elements of *L* for *i* = 1, 2 and *j* = 1, 2, 3, such that λ_{11}, λ_{12}, and λ_{13} are linearly independent over the rational numbers, and λ_{11} and λ_{21} are also linearly independent over the rational numbers, then the matrix

has rank 2.

## History

A special case of the result where *x*_{1}, *x*_{2}, and *x*_{3} are logarithms of positive integers, *y*_{1} = 1, and *y*_{2} is real, was first mentioned in a paper by Leonidas Alaoglu and Paul Erdős from 1944 in which they try to prove that the ratio of consecutive colossally abundant numbers is always prime. They claimed that Carl Ludwig Siegel knew of a proof of this special case, but it is not recorded.^{[1]} Using the special case they manage to prove that the ratio of consecutive colossally abundant numbers is always either a prime or a semiprime.

The theorem was first explicitly stated and proved in its complete form independently by Serge Lang^{[2]} and Kanakanahalli Ramachandra^{[3]} in the 1960s.

## Five exponentials theorem

A stronger, related result is the **five exponentials theorem**,^{[4]} which is as follows. Let *x*_{1}, *x*_{2} and *y*_{1}, *y*_{2} be two pairs of complex numbers, with each pair being linearly independent over the rational numbers, and let γ be a non-zero algebraic number. Then at least one of the following five numbers is transcendental:

This theorem implies the six exponentials theorem and in turn is implied by the as yet unproven four exponentials conjecture, which says that in fact one of the first four numbers on this list must be transcendental.

## Sharp six exponentials theorem

Another related result that implies both the six exponentials theorem and the five exponentials theorem is the **sharp six exponentials theorem**.^{[5]} This theorem is as follows. Let *x*_{1}, *x*_{2}, and *x*_{3} be complex numbers that are linearly independent over the rational numbers, and let *y*_{1} and *y*_{2} be a pair of complex numbers that are linearly independent over the rational numbers, and suppose that β_{ij} are six algebraic numbers for 1 ≤ *i* ≤ 3 and 1 ≤ *j* ≤ 2 such that the following six numbers are algebraic:

Then *x*_{i} *y*_{j} = β_{ij} for 1 ≤ *i* ≤ 3 and 1 ≤ *j* ≤ 2. The six exponentials theorem then follows by setting β_{ij} = 0 for every *i* and *j*, while the five exponentials theorem follows by setting *x*_{3} = γ/*x*_{1} and using Baker's theorem to ensure that the *x*_{i} are linearly independent.

There is a sharp version of the five exponentials theorem as well, although it as yet unproven so is known as the **sharp five exponentials conjecture**.^{[6]} This conjecture implies both the sharp six exponentials theorem and the five exponentials theorem, and is stated as follows. Let *x*_{1}, *x*_{2} and *y*_{1}, *y*_{2} be two pairs of complex numbers, with each pair being linearly independent over the rational numbers, and let α, β_{11}, β_{12}, β_{21}, β_{22}, and γ be six algebraic numbers with γ ≠ 0 such that the following five numbers are algebraic:

Then *x*_{i} *y*_{j} = β_{ij} for 1 ≤ *i*, *j* ≤ 2 and γ*x*_{2} = α*x*_{1}.

A consequence of this conjecture that isn't currently known would be the transcendence of *e*^{π²}, by setting *x*_{1} = *y*_{1} = β_{11} = 1, *x*_{2} = *y*_{2} = *i*π, and all the other values in the statement to be zero.

## Strong six exponentials theorem

A further strengthening of the theorems and conjectures in this area are the strong versions. The **strong six exponentials theorem** is a result proved by Damien Roy that implies the sharp six exponentials theorem.^{[7]} This result concerns the vector space over the algebraic numbers generated by 1 and all logarithms of algebraic numbers, denoted here as *L*^{∗}. So *L*^{∗} is the set of all complex numbers of the form

for some *n* ≥ 0, where all the β_{i} and α_{i} are algebraic and every branch of the logarithm is considered. The strong six exponentials theorem then says that if *x*_{1}, *x*_{2}, and *x*_{3} are complex numbers that are linearly independent over the algebraic numbers, and if *y*_{1} and *y*_{2} are a pair of complex numbers that are also linearly independent over the algebraic numbers then at least one of the six numbers *x*_{i} *y*_{j} for 1 ≤ *i* ≤ 3 and 1 ≤ *j* ≤ 2 is not in *L*^{∗}. This is stronger than the standard six exponentials theorem which says that one of these six numbers is not simply the logarithm of an algebraic number.

There is also a **strong five exponentials conjecture** formulated by Michel Waldschmidt^{[8]} It would imply both, the strong six exponentials theorem and the sharp five exponentials conjecture. This conjecture claims that if *x*_{1}, *x*_{2} and *y*_{1}, *y*_{2} are two pairs of complex numbers, with each pair being linearly independent over the algebraic numbers, then at least one of the following five numbers is not in *L*^{∗}:

All the above conjectures and theorems are consequences of the unproven extension of Baker's theorem, that logarithms of algebraic numbers that are linearly independent over the rational numbers are automatically algebraically independent too. The diagram on the right shows the logical implications between all these results.

## Generalization to commutative group varieties

The exponential function uniformizes the exponential map of the multiplicative group . The six exponential theorem can be therefore reformulated in a more abstract fashion:

Let be the field of complex numbers and let . Let → be a non-zero complex-analytic group homomorphism. Denote by the group of numbers in , such that is an algebraic point of . If can be only generated by more than two elements over the field of rational numbers , then the image is an algebraic subgroup of .

In this way, the statement of the six exponentials theorem can be generalized to an arbitrary commutative group variety over the field of algebraic numbers. Alternatively, one can replace by
and "more than two elements" by "more than one element" to obtain another variant of the generalization. This **generalized six exponential conjecture**, however, seems out of scope at the current state of transcendental number theory.

For the special, but interesting cases × and × , with elliptic curves over the field of algebraic numbers, results towards the generalized six exponential conjecture could be established by Aleksander Momot.^{[9]} These results involve the exponential function and a Weierstrass function resp. two Weierstrass functions with algebraic invariants , instead of two exponential functions as in the classical statement. In the classical statement, the numbers play the role of a generating set of , that is, .

For an algebraic group × it is proved in,^{[10]} among others, that if is not isogenous to a curve over a real field and if is not an algebraic subgroup of , then can be either generated by two elements over , or a minimal generating set of over consists of three elements which are not all contained in a real line ( a non-zero complex number). A similar result is shown for
× .

## Notes

- ↑ Alaoglu and Erdős, (1944), p.455: "Professor Siegel has communicated to us the result that
*q*^{ x},*r*^{ x}and*s*^{ x}can not be simultaneously rational except if*x*is an integer." - ↑ Lang, (1966), chapter 2, section 1.
- ↑ Ramachandra, (1967/68).
- ↑ Waldschmidt, (1988), corollary 2.2.
- ↑ Waldschmidt, (2005), theorem 1.4.
- ↑ Waldschmidt, (2005), conjecture 1.5
- ↑ Roy, (1992), section 4, corollary 2.
- ↑ Waldschmidt, (1988).
- ↑ Momot, ch. 7
- ↑ Momot, ch. 7

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