21661253 has 2 divisors, whose sum is σ = 21661254. Its totient is φ = 21661252.

The previous prime is 21661223. The next prime is 21661271. The reversal of 21661253 is 35216612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 15194404 + 6466849 = 3898^2 + 2543^2 .

It is a cyclic number.

It is not a de Polignac number, because 21661253 - 2¹² = 21657157 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21661223) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10830626 + 10830627.

It is an arithmetic number, because the mean of its divisors is an integer number (10830627).

Almost surely, 2^21661253 is an apocalyptic number.

It is an amenable number.

21661253 is a deficient number, since it is larger than the sum of its proper divisors (1).

21661253 is an equidigital number, since it uses as much as digits as its factorization.

21661253 is an evil number, because the sum of its binary digits is even.

The product of its digits is 2160, while the sum is 26.

The square root of 21661253 is about 4654.1651238434. The cubic root of 21661253 is about 278.7583327156.

Adding to 21661253 its reverse (35216612), we get a palindrome (56877865).

The spelling of 21661253 in words is "twenty-one million, six hundred sixty-one thousand, two hundred fifty-three".