21742561 has 8 divisors (see below), whose sum is σ = 22247784. Its totient is φ = 21240960.

The previous prime is 21742493. The next prime is 21742571. The reversal of 21742561 is 16524712.

It can be written as a sum of positive squares in 4 ways, for example, as 1806336 + 19936225 = 1344^2 + 4465^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21742561 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21742571) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 13941 + ... + 15421.

It is an arithmetic number, because the mean of its divisors is an integer number (2780973).

Almost surely, 2^21742561 is an apocalyptic number.

It is an amenable number.

21742561 is a deficient number, since it is larger than the sum of its proper divisors (505223).

21742561 is a wasteful number, since it uses less digits than its factorization.

21742561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1811.

The product of its digits is 3360, while the sum is 28.

The square root of 21742561 is about 4662.8919138234. The cubic root of 21742561 is about 279.1066810251.

It can be divided in two parts, 217 and 42561, that added together give a triangular number (42778 = T₂₉₂).

The spelling of 21742561 in words is "twenty-one million, seven hundred forty-two thousand, five hundred sixty-one".