816561 has 20 divisors (see below), whose sum is σ = 1293732. Its totient is φ = 511488.

The previous prime is 816559. The next prime is 816581. The reversal of 816561 is 165618.

816561 is a `hidden beast` number, since 8 + 1 + 656 + 1 = 666.

It can be written as a sum of positive squares in 2 ways, for example, as 571536 + 245025 = 756^2 + 495^2 .

It is not a de Polignac number, because 816561 - 2¹ = 816559 is a prime.

It is a super-3 number, since 3×816561³ = 1633379706784699443, which contains 333 as substring.

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

It is an alternating number because its digits alternate between even and odd.

It is a Curzon number.

It is a plaindrome in base 12.

It is not an unprimeable number, because it can be changed into a prime (816521) 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 19 ways as a sum of consecutive naturals, for example, 1081 + ... + 1673.

2⁸¹⁶⁵⁶¹ is an apocalyptic number.

816561 is a gapful number since it is divisible by the number (81) formed by its first and last digit.

It is an amenable number.

816561 is a deficient number, since it is larger than the sum of its proper divisors (477171).

816561 is a wasteful number, since it uses less digits than its factorization.

816561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 622 (or 613 counting only the distinct ones).

The product of its digits is 1440, while the sum is 27.

The square root of 816561 is about 903.6376486181. The cubic root of 816561 is about 93.4679845014.

It can be divided in two parts, 81 and 6561, that multiplied together give a 12-th power (531441 = 3¹²).

The spelling of 816561 in words is "eight hundred sixteen thousand, five hundred sixty-one".