317

317.

Let's list the above number in ascending order by taking one digit, two digits, and three digits at a time, respectively.

One digit numbers: 1,3,7

Two digit numbers  : 13,17,31,37,71,73

Three digit numbers: 137,173,317,371,713,731.

Now here comes the beauty. Numbers that are less than 317 (including 317) are prime (the number 1 is considered to be prime here in this context)! Numbers greater than 317 are composite! But what is so special about the number 317?

That's my hostel room number :P.

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