21661001 has 2 divisors, whose sum is σ = 21661002. Its totient is φ = 21661000.

The previous prime is 21660967. The next prime is 21661027. The reversal of 21661001 is 10016612.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19811401 + 1849600 = 4451^2 + 1360^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21661001 is a prime.

It is not a weakly prime, because it can be changed into another prime (21661051) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10830500 + 10830501.

It is an arithmetic number, because the mean of its divisors is an integer number (10830501).

Almost surely, 2^21661001 is an apocalyptic number.

It is an amenable number.

21661001 is a deficient number, since it is larger than the sum of its proper divisors (1).

21661001 is an equidigital number, since it uses as much as digits as its factorization.

21661001 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 72, while the sum is 17.

The square root of 21661001 is about 4654.1380512400. The cubic root of 21661001 is about 278.7572517167.

Adding to 21661001 its reverse (10016612), we get a palindrome (31677613).

The spelling of 21661001 in words is "twenty-one million, six hundred sixty-one thousand, one".