821685661 has 2 divisors, whose sum is σ = 821685662. Its totient is φ = 821685660.

The previous prime is 821685643. The next prime is 821685749. The reversal of 821685661 is 166586128.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 718561636 + 103124025 = 26806^2 + 10155^2 .

It is a cyclic number.

It is not a de Polignac number, because 821685661 - 2⁹ = 821685149 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (821685061) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 410842830 + 410842831.

It is an arithmetic number, because the mean of its divisors is an integer number (410842831).

Almost surely, 2^821685661 is an apocalyptic number.

It is an amenable number.

821685661 is a deficient number, since it is larger than the sum of its proper divisors (1).

821685661 is an equidigital number, since it uses as much as digits as its factorization.

821685661 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 138240, while the sum is 43.

The square root of 821685661 is about 28665.0599336544. The cubic root of 821685661 is about 936.6310900455.

The spelling of 821685661 in words is "eight hundred twenty-one million, six hundred eighty-five thousand, six hundred sixty-one".