21081551 has 2 divisors, whose sum is σ = 21081552. Its totient is φ = 21081550.

The previous prime is 21081517. The next prime is 21081559. The reversal of 21081551 is 15518012.

It is a strong prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21081551 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21081559) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10540775 + 10540776.

It is an arithmetic number, because the mean of its divisors is an integer number (10540776).

Almost surely, 2^21081551 is an apocalyptic number.

21081551 is a deficient number, since it is larger than the sum of its proper divisors (1).

21081551 is an equidigital number, since it uses as much as digits as its factorization.

21081551 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 400, while the sum is 23.

The square root of 21081551 is about 4591.4650167457. The cubic root of 21081551 is about 276.2490881265.

Adding to 21081551 its reverse (15518012), we get a palindrome (36599563).

The spelling of 21081551 in words is "twenty-one million, eighty-one thousand, five hundred fifty-one".