841

841 = 29 x 29.

841 is a centered square number. It is also a centered heptagonal number and a centered octagonal number.

841 has a representation as a sum of two squares: 841 = 20² + 21². It is the sum of two consecutive squares.

841 is the hypotenuse of a primitive Pythagorean triple: 841² = 41² + 840².

841 is the smallest 3-digit square whose digits are decreasing.

841 is the smallest binary prime square. It's a perfect square whose binary representation (1101001001) considered as a decimal is a prime number.

841 is a divisor of 41⁴ - 1.

Source: Number Gossip