21121561 has 4 divisors (see below), whose sum is σ = 21692452. Its totient is φ = 20550672.

The previous prime is 21121547. The next prime is 21121603. The reversal of 21121561 is 16512112.

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 19855936 + 1265625 = 4456^2 + 1125^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21121561 is a prime.

It is a super-2 number, since 2×21121561² = 892240678153442, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21121501) 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, 285390 + ... + 285463.

It is an arithmetic number, because the mean of its divisors is an integer number (5423113).

Almost surely, 2^21121561 is an apocalyptic number.

It is an amenable number.

21121561 is a deficient number, since it is larger than the sum of its proper divisors (570891).

21121561 is an equidigital number, since it uses as much as digits as its factorization.

21121561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 570890.

The product of its digits is 120, while the sum is 19.

The square root of 21121561 is about 4595.8199486055. The cubic root of 21121561 is about 276.4237391152.

Adding to 21121561 its reverse (16512112), we get a palindrome (37633673).

The spelling of 21121561 in words is "twenty-one million, one hundred twenty-one thousand, five hundred sixty-one".