821561 has 4 divisors (see below), whose sum is σ = 884772. Its totient is φ = 758352.

The previous prime is 821551. The next prime is 821573. The reversal of 821561 is 165128.

821561 = T₃₄₉ + T₃₅₀ + ... + T₃₆₁.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 525625 + 295936 = 725^2 + 544^2 .

It is a cyclic number.

It is not a de Polignac number, because 821561 - 2⁶ = 821497 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (821551) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 31586 + ... + 31611.

It is an arithmetic number, because the mean of its divisors is an integer number (221193).

2⁸²¹⁵⁶¹ is an apocalyptic number.

It is an amenable number.

821561 is a deficient number, since it is larger than the sum of its proper divisors (63211).

821561 is a wasteful number, since it uses less digits than its factorization.

821561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 63210.

The product of its digits is 480, while the sum is 23.

The square root of 821561 is about 906.4000220653. The cubic root of 821561 is about 93.6583721050.

Adding to 821561 its reverse (165128), we get a palindrome (986689).

The spelling of 821561 in words is "eight hundred twenty-one thousand, five hundred sixty-one".