11

This number is a prime.

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11 is the smallest prime of the form n ns followed by 1. Note that eleven 11s followed by 1 is also prime. [Poo Sung]

The first prime which cannot be a pandigital fraction (including or excluding 0). [Poo Sung]

11 = 1₂ + 11₂ + 111₂. [Poo Sung]

"Mrs. Perkins' Quilt" is a puzzle to dissect a square of side n into a number of smaller squares of integral sides. Note that 11 squares are the smallest for a square of side 11. [Poo Sung]