21001011661 has 4 divisors (see below), whose sum is σ = 24001156192. Its totient is φ = 18000867132.

The previous prime is 21001011653. The next prime is 21001011667. The reversal of 21001011661 is 16611010012.

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 21001011661 - 2³ = 21001011653 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21001011667) 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, 1500072255 + ... + 1500072268.

It is an arithmetic number, because the mean of its divisors is an integer number (6000289048).

Almost surely, 2^21001011661 is an apocalyptic number.

It is an amenable number.

21001011661 is a deficient number, since it is larger than the sum of its proper divisors (3000144531).

21001011661 is an equidigital number, since it uses as much as digits as its factorization.

21001011661 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3000144530.

The product of its (nonzero) digits is 72, while the sum is 19.

Adding to 21001011661 its reverse (16611010012), we get a palindrome (37612021673).

The spelling of 21001011661 in words is "twenty-one billion, one million, eleven thousand, six hundred sixty-one".