I am a postdoctoral student at the Max Planck Institute for Mathematics in Bonn. You can find me at the MPIM until August 2021, after which I will move to Ann Arbor, Michigan. I completed my PhD in Leiden under the supervision of Jan-Hendrik Evertse and Peter Stevenhagen. The PhD thesis is freely available here. I am interested in number theory and surrounding areas, in particular the distribution of class groups, the circle method and Diophantine equations.

- A sharp upper bound for the 2-torsion of class groups of multiquadratic fields (with C. Pagano), arXiv, submitted.
- Higher Rédei reciprocity and integral points on conics (with C. Pagano), arXiv.
- Effective convergence of coranks of random Rédei matrices (with C. Pagano), arXiv.
- On the 4-rank of class groups of Dirichlet biquadratic fields (with E. Fouvry and C. Pagano), arXiv, submitted.
- On Dirichlet biquadratic fields (with E. Fouvry), arXiv, submitted.
- Higher genus theory (with C. Pagano), International Mathematics Research Notices, To Appear. [arXiv:1909.13871]
- On the negative Pell equation (with S. Chan, D. Milovic and C. Pagano), arXiv.
- On the distribution of Cl(K)[l^infty] for degree l cyclic fields (with C. Pagano), arXiv, submitted.
- Joint distribution of spins (with D. Milovic), arXiv, submitted.
- The 16-rank of Q(-p), Algebra and Number Theory, 14(2020), 37-65. [arXiv:1809.07167]
- Unit equations and Fermat surfaces in positive characteristic (with C. Pagano), Acta Arithmetica, 193(2020), 133-156. [arXiv:1806.01614]
- Vinogradov's three primes theorem with primes having given primitive roots (with C. Frei and E. Sofos), Mathematical Proceedings of the Cambridge Philosophical Society, To Appear. [arXiv:1804.00573]
- Spins of prime ideals and the negative Pell equation x^2 - 2py^2 = -1 (with D. Milovic), Compositio Mathematica, 155(2019), 100-125. [arXiv:1611.10337]
- On the 16-rank of class groups of Q(-2p) for primes p = 1 mod 4 (with D. Milovic), International Mathematics Research Notices, 23(2019), 7406-7427. [arXiv:1705.05505]
- On the equation x_1 + x_2 = 1 in finitely generated multiplicative groups in positive characteristic (with C. Pagano), The Quarterly Journal of Mathematics, 68(2017), 923-934. [arXiv:1610.08377]
- The generalized Catalan equation in positive characteristic, arXiv, submitted.
- The Catalan equation, Indagationes Mathematicae, 28(2017), 321-352. [arXiv:1610.06440]

- The Catalan equation, Algebra, Geometry and Number Theory Seminar, Leiden, 22 June 2015
- An application of Vinogradov's method to class groups, Diamant symposium, Soest, 25 November 2016
- Vinogradov's method for exponential sums and applications, Analytic Number Theory Seminar, Leiden, 12 December 2016
- On the equation x + y = 1 in finitely generated multiplicative groups in positive characteristic, Intercity Number Theory Seminar, Leiden, 10 March 2017
- De laatste stelling van Fermat voor C[t], Bachelorseminarium Algebra, meetkunde en getaltheorie, Leiden, 10 May 2017
- Unit equations in characteristic p, Journées Arithmétiques, Caen, 3 July 2017
- On the 16-rank of Q(-p), Seminar, London, 20 November 2017
- Ternary Goldbach for Artin primes, Diamant symposium, Breukelen, 1 December 2017
- A survey on 2-parts of class groups, Seminar, Bonn, 27 March 2018
- Vinogradov's method and class groups, ELAZ, Bonn, 6 September 2018
- The spin of prime ideals and applications, Seminar, Copenhagen, 8 January 2019
- The spin of prime ideals and applications, Seminar, Paris, 14 January 2019
- De laatste stelling van Fermat voor C[t], Bachelorseminarium Algebra, meetkunde en getaltheorie, Leiden, 13 February 2019
- The generalized Catalan equation in positive characteristic, Diophantine seminar, Amsterdam, 11 April 2019
- Factoring in number rings, NMC, Veldhoven, 24 April 2019
- On Gerth's conjectures, Diamant symposium, Veldhoven, 25 April 2019
- The spin of prime ideals and applications, Intercity Number Theory Seminar, Leiden, 10 May 2019
- Factoring in number rings, This Week's Discoveries, Leiden, 21 May 2019
- Spins, governing fields and negative Pell, Enumerative Arithmetic and the Cohen-Lenstra heuristics, Bonn, 7 June 2019
- On Chowla's conjecture over function fields, Seminar, Cologne, 26 November 2019
- On Chowla's conjecture over function fields, Intercity Number Theory Seminar, Leiden, 7 February 2020
- The negative Pell equation and the 8-rank of the class group, Tel Aviv number theory seminar, Online, 4 June 2020
- Integral points on quadratic equations, Michigan - Arithmetic Geometry Initiative - Columbia, Online, 18 June 2020
- The negative Pell equation, governing fields and beyond, Number Theory Lunch Seminar, Online, 16 September 2020

- ANA2015, Delft, 28 - 30 October 2015
- Diamant symposium, Lunteren, 26 - 27 November 2015
- Diamant symposium, Veenendaal, 26 - 27 May 2016
- Diamant symposium, Soest, 24 - 25 November 2016
- Diamant symposium, Breukelen, 1 - 2 June 2017
- Journées Arithmétiques, Caen, 3 - 7 July 2017
- Diamant symposium, Breukelen, 30 November 2017 - 1 December 2017
- Diamant symposium, Veldhoven, 4 - 5 April 2018
- Effective Methods for Diophantine Problems, Leiden, 18 - 22 June 2018
- ELAZ, Bonn, 3 - 7 September 2018
- NMC, Veldhoven, 23 - 24 April 2019
- Diamant symposium, Veldhoven, 24 - 25 April 2019
- Enumerative Arithmetic and the Cohen-Lenstra heuristics, Bonn, 3 - 7 June 2019
- Local-Global Principles and Arithmetic Statistics, Vienna, Postponed

MasterMath Elliptic curves (Fall 2015)

Algebra 1 (Spring 2016)

Lineaire Algebra 1 (Fall 2016)

MasterMath Elliptic curves (Spring 2017)

Diophantine approximation (Fall 2017)

Algebra 1 (Spring 2018)

Analytic Number Theory (Fall 2018)

Intercity Number Theory Seminar (continuously)

General Mathematics Colloquium (continuously)

Knot Theory (Spring 2016)

Analytic Number Theory Seminar (Summer 2016 and Winter 2017)

Obstructions to local-global principles (Spring 2017)

Seminar on the work of Alexander Smith (Fall 2018)

Diophantine Seminar

PLEASANT Seminar (Summer 2019 and Winter 2020)

The Catalan equation (master thesis)

MPIM Office 221

E-mail: koymans