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