21291661 has 2 divisors, whose sum is σ = 21291662. Its totient is φ = 21291660.

The previous prime is 21291643. The next prime is 21291671. The reversal of 21291661 is 16619212.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 12823561 + 8468100 = 3581^2 + 2910^2 .

It is a cyclic number.

It is not a de Polignac number, because 21291661 - 2¹¹ = 21289613 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21291671) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10645830 + 10645831.

It is an arithmetic number, because the mean of its divisors is an integer number (10645831).

Almost surely, 2^21291661 is an apocalyptic number.

It is an amenable number.

21291661 is a deficient number, since it is larger than the sum of its proper divisors (1).

21291661 is an equidigital number, since it uses as much as digits as its factorization.

21291661 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 1296, while the sum is 28.

The square root of 21291661 is about 4614.2887859344. The cubic root of 21291661 is about 277.1638046382.

It can be divided in two parts, 21291 and 661, that added together give a cube (21952 = 28³).

The spelling of 21291661 in words is "twenty-one million, two hundred ninety-one thousand, six hundred sixty-one".