821691 has 16 divisors (see below), whose sum is σ = 1311520. Its totient is φ = 505440.

The previous prime is 821677. The next prime is 821741. The reversal of 821691 is 196128.

It is not a de Polignac number, because 821691 - 2¹³ = 813499 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a Duffinian number.

821691 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (821641) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 820 + ... + 1521.

It is an arithmetic number, because the mean of its divisors is an integer number (81970).

2⁸²¹⁶⁹¹ is an apocalyptic number.

821691 is a deficient number, since it is larger than the sum of its proper divisors (489829).

821691 is a wasteful number, since it uses less digits than its factorization.

821691 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2363 (or 2357 counting only the distinct ones).

The product of its digits is 864, while the sum is 27.

The square root of 821691 is about 906.4717314953. The cubic root of 821691 is about 93.6633118665.

The spelling of 821691 in words is "eight hundred twenty-one thousand, six hundred ninety-one".