21151661 has 2 divisors, whose sum is σ = 21151662. Its totient is φ = 21151660.

The previous prime is 21151639. The next prime is 21151673. The reversal of 21151661 is 16615112.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21114025 + 37636 = 4595^2 + 194^2 .

It is a cyclic number.

It is not a de Polignac number, because 21151661 - 2¹⁰ = 21150637 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 (21151061) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10575830 + 10575831.

It is an arithmetic number, because the mean of its divisors is an integer number (10575831).

Almost surely, 2^21151661 is an apocalyptic number.

It is an amenable number.

21151661 is a deficient number, since it is larger than the sum of its proper divisors (1).

21151661 is an equidigital number, since it uses as much as digits as its factorization.

21151661 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 360, while the sum is 23.

The square root of 21151661 is about 4599.0934976362. The cubic root of 21151661 is about 276.5549858109.

Adding to 21151661 its reverse (16615112), we get a palindrome (37766773).

It can be divided in two parts, 21151 and 661, that added together give a palindrome (21812).

The spelling of 21151661 in words is "twenty-one million, one hundred fifty-one thousand, six hundred sixty-one".