In number theory, Skewes's number is any of several large numbers used by the South African mathematician Stanley Skewes as upper bounds for the smallest natural number for which where is the prime-counting function and is the logarithmic integral function. Skewes's number is much larger, but it is now known that there is a crossing between and near It is not known whether it is the smallest crossing. Skewes's numbers J.E. Littlewood, who was Skewes's research supervisor, had proved in that there is such a number (and so, a first such number); and indeed found that the sign of the difference changes infinitely many times. All numerical evidence then available seemed to suggest that was always less than Littlewood's proof did not, however, exhibit a concrete such number . proved that, assuming that the Riemann hypothesis is true, there exists a number violating below In , without assuming the Riemann hypothesis, Skewes proved that there must exist a value of below Skewes's task was to make Littlewood's existence proof effective: exhibiting some concrete upper bound for the first sign change. According to Georg Kreisel, this was at the time not considered obvious even in principle. More recent estimates These upper bounds have since been reduced considerably by using large-scale computer calculations of zeros of the Riemann zeta function. The first estimate for the actual value of a crossover point was given by , who showed that somewhere between and there are more than consecutive integers with . Without assuming the Riemann hypothesis, proved an upper bound of . A better estimate was discovered by , who showed there are at least consecutive integers somewhere near this value where . Bays and Hudson found a few much smaller values of where gets close to ; the possibility that there are crossover points near these values does not seem to have been definitely ruled out yet, though computer calculations suggest they are unlikely to exist. gave a small improvement and correction to the result of Bays and Hudson. found a smaller interval for a crossing, which was slightly improved by . The same source shows that there exists a number violating below . This can be reduced to assuming the Riemann hypothesis. gave . Rigorously, proved that there are no crossover points below , improved by to , by to , by to , and by to . There is no explicit