21315661 has 2 divisors, whose sum is σ = 21315662. Its totient is φ = 21315660.

The previous prime is 21315649. The next prime is 21315689. The reversal of 21315661 is 16651312.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 10850436 + 10465225 = 3294^2 + 3235^2 .

It is a cyclic number.

It is not a de Polignac number, because 21315661 - 2¹³ = 21307469 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21315611) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10657830 + 10657831.

It is an arithmetic number, because the mean of its divisors is an integer number (10657831).

Almost surely, 2^21315661 is an apocalyptic number.

It is an amenable number.

21315661 is a deficient number, since it is larger than the sum of its proper divisors (1).

21315661 is an equidigital number, since it uses as much as digits as its factorization.

21315661 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 1080, while the sum is 25.

The square root of 21315661 is about 4616.8886709558. Note that the first 3 decimals coincide. The cubic root of 21315661 is about 277.2679053863.

Adding to 21315661 its reverse (16651312), we get a palindrome (37966973).

The spelling of 21315661 in words is "twenty-one million, three hundred fifteen thousand, six hundred sixty-one".