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r) Elliptic curve rank values. In 2019, Klagsbrun, Sherman, and Weigandt used GRH for $L$-functions of elliptic curves and for zeta-functions of number fields to prove that the elliptic curve found by Elkies in 2006 with 28 independent rational points has rank equal to 28.

r) Elliptic curve rank values. In 2019, Klagsbrun, Sherman, and Weigandt used GRH for $L$-functions of elliptic curves and for zeta-functions of number fields to prove that the elliptic curve over $\mathbf Q$ found by Elkies in 2006 with 28 independent rational points has rank 28.

Update: In 2024, Elkies and Klagsbrun announced the discovery of an elliptic curve over $\mathbf Q$ with 29 independent rational points, and its rank is 29 assuming GRH. See here.

p) Proof of Andre-Oort conjecture. In 2014, Klingler and Yafaev showed GRH for zeta-functions of CM number fields implies the Andre-Oort conjecture. Daw and Orr gave another proof also using the same version of GRH.

p) Proof of André-Oort conjecture. In 2014, Klingler and Yafaev showed GRH for zeta-functions of CM number fields implies the André-Oort conjecture. Daw and Orr gave another proof also using the same version of GRH.

Update: This is now an obsolete application of GRH since the André-Oort conjecture has been proved without GRH. See here.

e) The Goldbach conjecture (1742). The "even" version says all even integers $n \geq 4$ are a sum of 2 primes, while the "odd" version says all odd integers $n \geq 7$ are a sum of 3 primes. For most mathematicians, the Goldbach conjecture is understood to mean the even version, and obviously the even version implies the odd version. There has been progress on the odd version if we assume GRH. In 1923, assuming all Dirichlet $L$-functions are nonzero in a right half-plane ${\text{Re}}(s) \geq 3/4 - \varepsilon$, where $\varepsilon$ is fixed (independent of the $L$-function), Hardy and Littlewood showed the odd Goldbach conjecture is true for all sufficiently large odd $n$. In 1937, Vinogradov proved the same result unconditionally, so he was able to remove GRH as a hypothesis. In 1997, Deshouillers, Effinger, te Riele, and Zinoviev showed the odd Goldbach conjecture is true for all odd $n \geq 7$ assuming GRH. That is, the odd Goldbach conjecture is completely settled if we use GRH.

Update: This is now an obsolete application of GRH since the odd Goldbach Conjecture was proved by Harald Helfgott in 2013 without appealing to GRH. An account of the current status of his work is here.

f) Polynomial-time primality tests. By results of Ankeny (1952) and Montgomery (1971), if GRH is true for all Dirichlet $L$-functions then the first nonmember of every proper subgroup of the unit group $({\mathbf Z}/m{\mathbf Z})^\times$ is $O((\log m)^2)$, where the $O$-constant is independent of $m$. In 1985, Bach showed with GRH for all Dirichlet $L$-functions that you take the constant in that $O$-estimate to be 2. That is, each proper subgroup of $({\mathbf Z}/m{\mathbf Z})^\times$ does not contain some integer from 1 to $2(\log m)^2$. Put differently, if a subgroup of $({\mathbf Z}/m{\mathbf Z})^\times$ contains all positive integers below $2(\log m)^2$ then the subgroup is the whole unit group mod $m$. (To understand one way that GRH has an influence on that upper bound, if the nontrivial zeros of all Dirichlet $L$-functions have ${\text{Re}}(s) \leq 1 - \varepsilon$ then the first nonmember of every proper subgroup of $({\mathbf Z}/m{\mathbf Z})^\times$ is $O((\log m)^{1/\varepsilon})$. Set $\varepsilon = 1/2$ to get the previous result I stated that uses GRH.) In 1976, Gary Miller introduced a deterministic primality test that he could prove runs in polynomial time using GRH for all Dirichlet $L$-functions. (Part of the test involves deciding if a subgroup of units mod $m$ is proper or not.) Shortly afterwards, Solovay and Strassen described a different primality test using Jacobi symbols. GRH for Dirichlet $L$-functions implies their test runs in polyomial time, and the subgroups of units mod $m$ occurring for their test contain $-1$, so the proof that their test runs in polynomial time "only" needs GRH for Dirichlet $L$-functions of even characters. (Solovay and Strassen described their test as a probabilistic test rather than a deterministic test, so they didn't mention GRH one way or the other.)

In 2002 Agrawal, Kayal, and Saxena created a new primality test that they could prove runs in polynomial time without using GRH anywhere in their argument. This is a nice example showing how GRH guides mathematicians in the direction of what should be true and then you hope to find a proof of those results by methods that don't involve assuming GRH.

g) Euclidean rings of integers. In 1973, Weinberger showed that if GRH is true for all Dedekind zeta-functions then every ring of algebraic integers with an infinite unit group (so ignoring $\mathbf Z$ and the ring of integers of imaginary quadratic fields) is Euclidean if it has class number 1. As a special case, in concrete terms, if $d$ is a positive integer that is not a perfect square then the ring ${\mathbf Z}[\sqrt{d}]$ is a unique factorization domain only if it is Euclidean. The same theorem is true for rings of $S$-integers: an infinite unit group plus class number $1$ plus GRH for zeta-functions of all number fields implies the ring is Euclidean. There has been progress in the direction of unconditional proofs that class number 1 implies Euclidean by Ram Murty and others, but as a striking special case let's consider ${\mathbf Z}[\sqrt{14}]$. It has class number 1 (which must have been known to Gauss in the early 19th century, in the language of quadratic forms), so it should be Euclidean. This particular real quadratic ring was first proved to be Euclidean only in 2004 (by M. Harper). So this is a ring that was known to have unique factorization for over 100 years before it was proved to be Euclidean.

h) Artin's primitive root conjecture. In 1927, Artin conjectured that each nonzero integer $a$ that is not $-1$ or a perfect square is a generator of $({\mathbf Z}/p{\mathbf Z})^\times$ for infinitely many primes $p$, and in fact for a positive proportion of such $p$. As a special case, taking $a = 10$, this says for primes $p$ the unit fraction $1/p$ has decimal period $p-1$ for a positive proportion of $p$. (For each prime $p$ other than $2$ and $5$, the decimal period for $1/p$ is a factor of $p-1$, so this special case is saying the largest possible period is realized infinitely often in a precise sense.) In 1967, Hooley showed Artin's primitive root conjecture follows from GRH for zeta-functions of number fields. More precisely, Artin's primitive root conjecture for $a$ follows from GRH for the zeta-functions of all the number fields $\mathbf Q(\sqrt[n]{a},\zeta_n)$ where $n$ runs over the squarefree positive integers. In 1984, R. Murty and Gupta showed without using GRH that Artin's primitive root conjecture is true for infinitely many $a$, but their proof couldn't pin down a specific $a$ for which the conjecture is true. In 1986, Heath-Brown showed without using GRH that Artin's primitive root conjecture is true for all prime values of $a$ with at most two exceptions (and of course there should not be any exceptions). Without using GRH, no definite $a$ is known for which Artin's conjecture is true.

m) Removing a condition in the Brauer-Siegel theorem. In 1947, Brauer proved the Brauer-Siegel theorem for sequences of number fields $K_n$ such that (i) $[K_n:\mathbf Q]/\log |{\rm disc}(K_n)| \to 0$ as $|{\rm disc}(K_n)| \to \infty$ and (ii) $K_n$ is Galois over $\mathbf Q$. If the zeta-functions $\zeta_{K_n}(s)$ all satisfy GRH (really, just no real zero in $(1/2,1)$) then we can drop condition (ii). That is, GRH implies the Brauer-Siegel theorem holds for sequences of number fields $K_n$ fitting condition (i).

o) Lower bounds on class numbers. In 1990, Louboutin showed GRH for zeta-functions of imaginary quadratic fields (really, the lack of real zeros in $(1/2,1)$ for these functions) implies the lower bound $h(\mathbf Q(\sqrt{-d})) \geq (\pi/(3e))\sqrt{d}/\log d$ for imaginary quadratic fields with discirminant $-d$. The point here is the explicit constant factor $\pi/(3e)$. (Hecke had shown such a lower bound with a "computable constant" $c$ in place of $\pi/(3e)$ but he did not compute the constant.) Without GRH, lower bounds for $h(\mathbf Q(\sqrt{-d}))$ are on the order of $\log d$, which is far smaller than $\sqrt{d}/\log d$. For example, Louboutin's GRH-based lower bound shows if $h(\mathbf Q(\sqrt{-d})) \leq 100$ then $d \leq $ 18,916,898. To put this 8-digit upper bound in perspective, when Watkins determined all imaginary quadratic fields with class number up to 100 in 2004, he used lower bounds on $h(\mathbf Q(\sqrt{-d}))$ that do not depend on GRH and the upper bound of his search space for $d$ was $e^{298368000}$, a number with 129,579,576 digits. Just the exponent in that upper bound on $d$ is greater than the upper bound on $d$ coming from GRH.

e) The Goldbach conjecture (1742). The "even" version says all even integers $n \geq 4$ are a sum of 2 primes, while the "odd" version says all odd integers $n \geq 7$ are a sum of 3 primes. For most mathematicians, the Goldbach conjecture is understood to mean the even version, and obviously the even version implies the odd version. The odd version turns out to be a consequence of GRH. In 1923, assuming all Dirichlet $L$-functions are nonzero in a right half-plane ${\text{Re}}(s) \geq 3/4 - \varepsilon$, where $\varepsilon$ is fixed (independent of the $L$-function), Hardy and Littlewood showed the odd Goldbach conjecture is true for all sufficiently large odd $n$. In 1937, Vinogradov proved the same result without needing GRH as a hypothesis. In 1997, Deshouillers, Effinger, te Riele, and Zinoviev showed GRH implies the odd Goldbach conjecture is true for all odd $n \geq 7$. So GRH completely settles the odd Goldbach conjecture.

Update: This is now an obsolete application of GRH since the odd Goldbach Conjecture was proved by Harald Helfgott in 2013 without needing GRH as a hypothesis. An account of the current status of Helfgott's work is here.

f) Polynomial-time primality tests. By results of Ankeny (1952) and Montgomery (1971), GRH for all Dirichlet $L$-functions implies that the first nonmember of every proper subgroup of the unit group $({\mathbf Z}/m{\mathbf Z})^\times$ is $O((\log m)^2)$, where the $O$-constant is independent of $m$. In 1985, Bach showed GRH for all Dirichlet $L$-functions implies the constant in that $O$-estimate can be taken to be 2. That is, GRH for all Dirichlet $L$-functions implies that each proper subgroup of $({\mathbf Z}/m{\mathbf Z})^\times$ is missing some integer from 1 to $2(\log m)^2$. Put differently, GRH for all Dirichlet $L$-functions implies that if a subgroup of $({\mathbf Z}/m{\mathbf Z})^\times$ contains all positive integers below $2(\log m)^2$ then the subgroup is the whole unit group mod $m$. (To understand one way that GRH has an influence on that upper bound, if all the nontrivial zeros of all Dirichlet $L$-functions have ${\text{Re}}(s) \leq 1 - \varepsilon$ then the first nonmember of every proper subgroup of $({\mathbf Z}/m{\mathbf Z})^\times$ is $O((\log m)^{1/\varepsilon})$. Set $\varepsilon = 1/2$ to get the previous result I stated that uses GRH.) In 1976, Gary Miller introduced a deterministic primality test that he could prove runs in polynomial time using GRH for all Dirichlet $L$-functions. (Part of the test involves deciding if a subgroup of units mod $m$ is proper or not.) Shortly afterwards, Solovay and Strassen described a different primality test using Jacobi symbols. GRH for Dirichlet $L$-functions implies their test runs in polyomial time, and the subgroups of units mod $m$ occurring for their test contain $-1$, so the proof that their test runs in polynomial time "only" needs GRH for Dirichlet $L$-functions of even characters. (Solovay and Strassen described their test as a probabilistic test rather than a deterministic test, so they didn't mention GRH one way or the other.)

In 2002 Agrawal, Kayal, and Saxena created a new primality test that they could prove runs in polynomial time without needing GRH. This is a nice example showing how GRH guides mathematicians in the direction of what should be true and then people tried to find a proof of those results by methods not involving GRH.

g) Euclidean rings of integers. In 1973, Weinberger showed that GRH for all Dedekind zeta-functions implies that every ring of algebraic integers with an infinite unit group (so ignoring $\mathbf Z$ and the ring of integers of imaginary quadratic fields) is Euclidean if it has class number 1. As a special case, in concrete terms, if $d$ is a positive integer that is not a perfect square then a ring ${\mathbf Z}[\sqrt{d}]$ that is a unique factorization domain must be Euclidean. The same theorem is true for rings of $S$-integers: an infinite unit group plus class number $1$ plus GRH for zeta-functions of all number fields implies the ring is Euclidean. Progress has been made by Ram Murty and others in the direction of proving class number 1 and infinite unit group implies Euclidean for rings of $S$-integers without needing GRH, and as a striking special case let's consider ${\mathbf Z}[\sqrt{14}]$. It has class number 1 (which must have been known to Gauss in the early 19th century, in the language of quadratic forms) and an infinite unit group, so it should be Euclidean. This particular real quadratic ring was first proved to be Euclidean only in 2004 (by M. Harper). So $\mathbf Z[\sqrt{14}]$ is a ring that was known to have unique factorization for over 100 years before it was proved to be Euclidean.

h) Artin's primitive root conjecture. In 1927, Artin conjectured that each nonzero integer $a$ that is not $-1$ or a perfect square is a generator of $({\mathbf Z}/p{\mathbf Z})^\times$ for infinitely many primes $p$, and in fact for a positive proportion of such $p$. As a special case, taking $a = 10$, this says for primes $p$ the unit fraction $1/p$ has decimal period $p-1$ for a positive proportion of $p$. (For each prime $p$ other than $2$ and $5$, the decimal period for $1/p$ is a factor of $p-1$, so this special case is saying the largest possible period is realized infinitely often in a precise sense.) In 1967, Hooley showed GRH for zeta-functions of number fields implies Artin's primitive root conjecture. More precisely, Artin's primitive root conjecture for $a$ follows from GRH for the zeta-functions of all the number fields $\mathbf Q(\sqrt[n]{a},\zeta_n)$ where $n$ runs over the squarefree positive integers. In 1984, R. Murty and Gupta showed Artin's primitive root conjecture is true for infinitely many $a$ without having to use GRH, but their proof couldn't pin down even one specific $a$ for which Artin's primitive root conjecture is true. In 1986, Heath-Brown showed without using GRH that Artin's primitive root conjecture is true for all prime values of $a$ with at most two exceptions (and of course there should not be any exceptions). Without using GRH, no definite $a$ is known for which Artin's conjecture is true.

m) Removing a condition in the Brauer-Siegel theorem. In 1947, Brauer proved the Brauer-Siegel theorem for sequences of number fields $K_n$ such that (i) $[K_n:\mathbf Q]/\log |{\rm disc}(K_n)| \to 0$ as $|{\rm disc}(K_n)| \to \infty$ and (ii) $K_n$ is Galois over $\mathbf Q$. If the zeta-functions $\zeta_{K_n}(s)$ all satisfy GRH (what is actually needed is no real zero in $(1/2,1)$ for those zeta-functions) then we can drop condition (ii). That is, GRH implies the Brauer-Siegel theorem holds for sequences of number fields $K_n$ fitting condition (i).

o) Lower bounds on class numbers. In 1990, Louboutin showed GRH for zeta-functions of imaginary quadratic fields (what is really needed is the lack of real zeros in $(1/2,1)$ for these zeta-functions) implies the lower bound $h(\mathbf Q(\sqrt{-d})) \geq (\pi/(3e))\sqrt{d}/\log d$ for imaginary quadratic fields with discirminant $-d$. The point here is the explicit constant factor $\pi/(3e)$. (Hecke had shown such a lower bound with a "computable constant" $c$ in place of $\pi/(3e)$ but he did not compute the constant.) Without GRH, lower bounds for $h(\mathbf Q(\sqrt{-d}))$ are on the order of $\log d$, which is far smaller than $\sqrt{d}/\log d$. For example, Louboutin's GRH-based lower bound shows if $h(\mathbf Q(\sqrt{-d})) \leq 100$ then $d \leq $ 18,916,898. To put this 8-digit upper bound in perspective, when Watkins determined all imaginary quadratic fields with class number up to 100 in 2004, he used lower bounds on $h(\mathbf Q(\sqrt{-d}))$ that do not depend on GRH and the upper bound of his search space for $d$ was $e^{298368000}$, a number with 129,579,576 digits. Just the exponent in that upper bound on $d$ is greater than the upper bound on $d$ coming from GRH.