21001640 has 64 divisors (see below), whose sum is σ = 52488000. Its totient is φ = 7498240.

The previous prime is 21001627. The next prime is 21001657. The reversal of 21001640 is 4610012.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 25556 + ... + 26364.

It is an arithmetic number, because the mean of its divisors is an integer number (820125).

Almost surely, 2^21001640 is an apocalyptic number.

21001640 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21001640, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (26244000).

21001640 is an abundant number, since it is smaller than the sum of its proper divisors (31486360).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21001640 is a wasteful number, since it uses less digits than its factorization.

21001640 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 890 (or 886 counting only the distinct ones).

The product of its (nonzero) digits is 48, while the sum is 14.

The square root of 21001640 is about 4582.7546301324. The cubic root of 21001640 is about 275.8995994125.

Adding to 21001640 its reverse (4610012), we get a palindrome (25611652).

It can be divided in two parts, 2100 and 1640, that multiplied together give a triangular number (3444000 = T₂₆₂₄).

The spelling of 21001640 in words is "twenty-one million, one thousand, six hundred forty".