21561251 has 2 divisors, whose sum is σ = 21561252. Its totient is φ = 21561250.

The previous prime is 21561223. The next prime is 21561277. The reversal of 21561251 is 15216512.

It is a happy number.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21561251 - 2¹⁴ = 21544867 is a prime.

It is not a weakly prime, because it can be changed into another prime (21561851) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10780625 + 10780626.

It is an arithmetic number, because the mean of its divisors is an integer number (10780626).

Almost surely, 2^21561251 is an apocalyptic number.

21561251 is a deficient number, since it is larger than the sum of its proper divisors (1).

21561251 is an equidigital number, since it uses as much as digits as its factorization.

21561251 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 600, while the sum is 23.

The square root of 21561251 is about 4643.4094155049. The cubic root of 21561251 is about 278.3286961431.

Adding to 21561251 its reverse (15216512), we get a palindrome (36777763).

It can be divided in two parts, 21561 and 251, that added together give a palindrome (21812).

The spelling of 21561251 in words is "twenty-one million, five hundred sixty-one thousand, two hundred fifty-one".