21001001 has 16 divisors (see below), whose sum is σ = 26521344. Its totient is φ = 16203264.

The previous prime is 21000983. The next prime is 21001021. The reversal of 21001001 is 10010012.

21001001 = T₁₈₈ + T₁₈₉ + ... + T₅₀₉.

It is a happy number.

It is not a de Polignac number, because 21001001 - 2¹⁰ = 20999977 is a prime.

It is a Duffinian number.

21001001 is a modest number, since divided by 1001 gives 21 as remainder.

It is not an unprimeable number, because it can be changed into a prime (21001021) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 1100 + ... + 6573.

It is an arithmetic number, because the mean of its divisors is an integer number (1657584).

Almost surely, 2^21001001 is an apocalyptic number.

It is an amenable number.

21001001 is a deficient number, since it is larger than the sum of its proper divisors (5520343).

21001001 is a wasteful number, since it uses less digits than its factorization.

21001001 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7720.

The product of its (nonzero) digits is 2, while the sum is 5.

The square root of 21001001 is about 4582.6849117084. The cubic root of 21001001 is about 275.8968011924.

Adding to 21001001 its reverse (10010012), we get a palindrome (31011013).

Multiplying 21001001 by its reverse (10010012), we get a palindrome (210220272022012).

It can be divided in two parts, 2 and 1001001, that multiplied together give a palindrome (2002002).

The spelling of 21001001 in words is "twenty-one million, one thousand, one".