21006561 has 16 divisors (see below), whose sum is σ = 29196288. Its totient is φ = 13419600.

The previous prime is 21006551. The next prime is 21006563. The reversal of 21006561 is 16560012.

It is a happy number.

It is not a de Polignac number, because 21006561 - 2¹¹ = 21004513 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21006563) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 8896 + ... + 11006.

It is an arithmetic number, because the mean of its divisors is an integer number (1824768).

Almost surely, 2^21006561 is an apocalyptic number.

It is an amenable number.

21006561 is a deficient number, since it is larger than the sum of its proper divisors (8189727).

21006561 is a wasteful number, since it uses less digits than its factorization.

21006561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2252.

The product of its (nonzero) digits is 360, while the sum is 21.

The square root of 21006561 is about 4583.2915028394. The cubic root of 21006561 is about 275.9211468710.

Adding to 21006561 its reverse (16560012), we get a palindrome (37566573).

The spelling of 21006561 in words is "twenty-one million, six thousand, five hundred sixty-one".