21681621 has 32 divisors (see below), whose sum is σ = 34872320. Its totient is φ = 13234752.

The previous prime is 21681619. The next prime is 21681643. The reversal of 21681621 is 12618612.

It is not a de Polignac number, because 21681621 - 2¹ = 21681619 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21681221) 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, 78135 + ... + 78411.

It is an arithmetic number, because the mean of its divisors is an integer number (1089760).

Almost surely, 2^21681621 is an apocalyptic number.

It is an amenable number.

21681621 is a deficient number, since it is larger than the sum of its proper divisors (13190699).

21681621 is a wasteful number, since it uses less digits than its factorization.

21681621 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 522 (or 516 counting only the distinct ones).

The product of its digits is 1152, while the sum is 27.

The square root of 21681621 is about 4656.3527572554. The cubic root of 21681621 is about 278.8456771713.

The spelling of 21681621 in words is "twenty-one million, six hundred eighty-one thousand, six hundred twenty-one".