21101661 has 32 divisors (see below), whose sum is σ = 35942400. Its totient is φ = 11985840.

The previous prime is 21101651. The next prime is 21101677. The reversal of 21101661 is 16610112.

21101661 is a `hidden beast` number, since 2 + 1 + 1 + 0 + 1 + 661 = 666.

It is not a de Polignac number, because 21101661 - 2¹¹ = 21099613 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21101651) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 58600 + ... + 58958.

It is an arithmetic number, because the mean of its divisors is an integer number (1123200).

Almost surely, 2^21101661 is an apocalyptic number.

21101661 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21101661 is a deficient number, since it is larger than the sum of its proper divisors (14840739).

21101661 is a wasteful number, since it uses less digits than its factorization.

21101661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 686 (or 680 counting only the distinct ones).

The product of its (nonzero) digits is 72, while the sum is 18.

The square root of 21101661 is about 4593.6544275772. The cubic root of 21101661 is about 276.3368995601.

Adding to 21101661 its reverse (16610112), we get a palindrome (37711773).

The spelling of 21101661 in words is "twenty-one million, one hundred one thousand, six hundred sixty-one".