21561261 has 4 divisors (see below), whose sum is σ = 28748352. Its totient is φ = 14374172.

The previous prime is 21561251. The next prime is 21561277. The reversal of 21561261 is 16216512.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21561261 - 2¹⁰ = 21560237 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21561251) 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, 3593541 + ... + 3593546.

It is an arithmetic number, because the mean of its divisors is an integer number (7187088).

Almost surely, 2^21561261 is an apocalyptic number.

It is an amenable number.

21561261 is a deficient number, since it is larger than the sum of its proper divisors (7187091).

21561261 is an equidigital number, since it uses as much as digits as its factorization.

21561261 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7187090.

The product of its digits is 720, while the sum is 24.

The square root of 21561261 is about 4643.4104922998. The cubic root of 21561261 is about 278.3287391722.

Adding to 21561261 its reverse (16216512), we get a palindrome (37777773).

The spelling of 21561261 in words is "twenty-one million, five hundred sixty-one thousand, two hundred sixty-one".