This thesis consists of an introduction and four papers. All four papers are devoted to problems in Number Theory. In Paper I, a special class of local ζ-functions is studied. The main theorem states that the functions have all zeros on the line Re(s)=1/2.This is a natural generalization of the result of Bump and Ng stating that the zeros of the Mellin transform of Hermite functions have Re(s)=1/2.In Paper II and Paper III we study eigenfunctions of desymmetrized quantized cat maps.If N denotes the inverse of Planck's constant, we show that the behavior of the eigenfunctions is very dependent on the arithmetic properties of N. If N is a square, then there are normalized eigenfunctions with supremum norm equal to <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?N%5E%7B1/4%7D" />, but if N is a prime, the supremum norm of all eigenfunctions is uniformly bounded. We prove the sharp estimate <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5C%7C%5Cpsi%5C%7C_%5Cinfty=O(N%5E%7B1/4%7D)" /> for all normalized eigenfunctions and all $N$ outside of a small exceptional set. For normalized eigenfunctions of the cat map (not necessarily desymmetrized), we also prove an entropy estimate and show that our functions satisfy equality in this estimate.We call a special class of eigenfunctions newforms and for most of these we are able to calculate their supremum norm explicitly.For a given <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?N=p%5Ek" />, with k&gt;1, the newforms can be divided in two parts (leaving out a small number of them in some cases), the first half all have supremum norm about <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?2/%5Csqrt%7B1%5Cpm%201/p%7D" /> and the supremum norm of the newforms in the second half have at most three different values, all of the order <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?N%5E%7B1/6%7D" />. The only dependence of A is that the normalization factor is different if A has eigenvectors modulo p or not. We also calculate the joint value distribution of the absolute value of n different newforms.In Paper IV we prove a generalization of Mertens' theorem to Beurling primes, namely that \lim_{n \to \infty}\frac{1}{\ln n}\prod_{p \leq n} \left(1-p^{-1}\right)^{-1}=Ae^{\gamma}<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Clim_%7Bn%20%5Cto%20%5Cinfty%7D%5Cfrac%7B1%7D%7B%5Cln%20n%7D%5Cprod_%7Bp%20%5Cleq%20n%7D%0A%5Cleft(1-p%5E%7B-1%7D%5Cright)%5E%7B-1%7D=Ae%5E%7B%5Cgamma%7D," />where γ is Euler's constant and Ax is the asymptotic number of generalized integers less than x. Thus the limit <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?M=%5Clim_%7Bn%5Cto%5Cinfty%7D%5Cleft(%5Csum_%7Bp%5Cle%20n%7Dp%5E%7B-1%7D-%5Cln(%5Cln%20n)%5Cright)" />exists. We also show that this limit coincides with <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Clim_%7B%5Calpha%5Cto%200%5E+%7D%0A%5Cleft(%5Csum_p%20p%5E%7B-1%7D(%5Cln%20p)%5E%7B-%5Calpha%7D-1/%5Calpha%5Cright)" /> ; for ordinary primes this claim is called Meissel's theorem. Finally we will discuss a problem posed by Beurling, namely how small |N(x)-[x] | can be made for a Beurling prime number system Q≠P, where P is the rational primes. We prove that for each c&gt;0 there exists a Q such that |N(x)-[x] | / QC 20100902