21506731405666 has 24 divisors (see below), whose sum is σ = 35505195668244. Its totient is φ = 9763459392960.

The previous prime is 21506731405651. The next prime is 21506731405699. The reversal of 21506731405666 is 66650413760512.

It can be written as a sum of positive squares in 6 ways, for example, as 64006470025 + 21442724935641 = 252995^2 + 4630629^2 .

It is an unprimeable number.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 521531701 + ... + 521572936.

Almost surely, 2^{21506731405666} is an apocalyptic number.

21506731405666 is a gapful number since it is divisible by the number (26) formed by its first and last digit.

21506731405666 is a deficient number, since it is larger than the sum of its proper divisors (13998464262578).

21506731405666 is a wasteful number, since it uses less digits than its factorization.

21506731405666 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1043104726 (or 1043104713 counting only the distinct ones).

The product of its (nonzero) digits is 5443200, while the sum is 52.

The spelling of 21506731405666 in words is "twenty-one trillion, five hundred six billion, seven hundred thirty-one million, four hundred five thousand, six hundred sixty-six".