21155113561 has 4 divisors (see below), whose sum is σ = 21198199824. Its totient is φ = 21112027300.

The previous prime is 21155113547. The next prime is 21155113579. The reversal of 21155113561 is 16531155112.

It is a happy number.

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 21155113561 - 2³¹ = 19007629913 is a prime.

It is a super-2 number, since 2×21155113561² (a number of 21 digits) contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21155113501) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 21542395 + ... + 21543376.

It is an arithmetic number, because the mean of its divisors is an integer number (5299549956).

Almost surely, 2^21155113561 is an apocalyptic number.

It is an amenable number.

21155113561 is a deficient number, since it is larger than the sum of its proper divisors (43086263).

21155113561 is an equidigital number, since it uses as much as digits as its factorization.

21155113561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 43086262.

The product of its digits is 4500, while the sum is 31.

Adding to 21155113561 its reverse (16531155112), we get a palindrome (37686268673).

The spelling of 21155113561 in words is "twenty-one billion, one hundred fifty-five million, one hundred thirteen thousand, five hundred sixty-one".