How does population requirement work in Civilization VI?

I've read that "Each 3 Population in a city allows it to support an additional District.", however I've seen requirements such as 3, 6, 15, and 19. So kinda goes off the rails with divisible by three. Anyone sure?

Note that the tooltips are off when I take screenshot - it is currently saying 10 pop required for theater.

While the law if three holds true in most cases, there are other factors that can influence the values e.g. some Civs get additional district bonuses etc.

To be a bit more explicit with what the comments and Cornelius has said.

Formula

P = 3(d - 1) + 1

• P is the number of population.
• d is the number of specialty districts built in the city.

Examples

1. 1st district requires 1 population.
2. 2nd district requires 4 population.
3. 3rd district requires 7 population
4. and so on...