## Connecting PPTP VPN in Linux

I am in CUHK. Hence I need to connect to VPN from time to time. The CUHK VPN pool is using PPTP implemented by the Cisco routers or access servers (although another using IPSec is also available, but I am not supposed to know about its existence…) and I am... [more]

## Stirling's Formula

We may need to calculate factorials $$n!$$ from time to time. But we will easily get into a very large number as $$n$$ increases. We all know Stirling’s formula for calculating factorials: [more]