21005001 has 10 divisors (see below), whose sum is σ = 31377962. Its totient is φ = 14003280.

The previous prime is 21004999. The next prime is 21005009. The reversal of 21005001 is 10050012.

It is a happy number.

It can be written as a sum of positive squares in only one way, i.e., 11696400 + 9308601 = 3420^2 + 3051^2 .

It is not a de Polignac number, because 21005001 - 2¹ = 21004999 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (9).

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21005009) by changing a digit.

It is a polite number, since it can be written in 9 ways as a sum of consecutive naturals, for example, 129580 + ... + 129741.

Almost surely, 2^21005001 is an apocalyptic number.

It is an amenable number.

21005001 is a deficient number, since it is larger than the sum of its proper divisors (10372961).

21005001 is an equidigital number, since it uses as much as digits as its factorization.

21005001 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 259333 (or 259324 counting only the distinct ones).

The product of its (nonzero) digits is 10, while the sum is 9.

The square root of 21005001 is about 4583.1213163084. The cubic root of 21005001 is about 275.9143165027.

Adding to 21005001 its reverse (10050012), we get a palindrome (31055013).

The spelling of 21005001 in words is "twenty-one million, five thousand, one".