Effective de la Valle Poussin style bounds on the first Chebyshev function

Abstract: In 1898 Charles Jean de la Valle Poussin, as part of his famed proof of the prime number theorem, developed an ineffective bound on the first Chebyshev function of the form: [ |\theta(x)-x| = \mathcal{O}\left(x \exp(-K \sqrt{\ln x})\right). ] This bound holds for $x$ sufficiently large, $x\geq x_0$, and $K$ some unspecified positive constant. To the best of my knowledge this bound has never been made effective -- I have never yet seen this bound made fully explicit, with precise values being given for $x_0$ and $K$. Herein, using a number of effective results established over the past 50 years, I shall develop two very simple explicit fully effective bounds of this type: [ |\theta(x)-x| < \; {x} \;\exp\left( - {1\over4} \sqrt{\ln x}\right); \qquad (x\geq 2). ] [ |\theta(x)-x| < \; {x} \;\exp\left( - {1\over3} \sqrt{\ln x}\right); \qquad (x\geq 3). ] Many other fully explicit bounds along these lines can easily be developed. For instance one can trade off stringency against range of validity: [ |\theta(x)-x| < \; {1\over 2} \; {x} \;\exp\left( - {1\over4} \sqrt{\ln x}\right); \qquad (x\geq 29), ] [ |\theta(x)-x| < \; {1\over 2} \; {x} \;\exp\left( - {1\over3} \sqrt{\ln x}\right); \qquad (x\geq 41). ] With hindsight, some of these effective bounds could have been established almost 50 years ago.