Almost perfect numbers refer to numbers where
σ(x) = 2x - 1, where σ is the sum of divisors function. Any number in the form 2n is almost perfect because
σ(2n) = 1 + 2 + 4 + ... + 2n = 2n+1-1 = 2(2n) - 1.
It is unknown whether any other almost perfect numbers exist.
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Yes. The only known almost perfect numbers are the powers of 2. 32 = 2^5 is an almost perfect number. It has not yet been proved whether {x: x = 2^n for n in N} = {x: x is an almost perfect number}.
No. The only known almost perfect numbers are the powers of 2, namely 1, 2, 4, 8, 16, 32, ...
There are infinitely many perfect numbers so they cannot all be listed.
There is a one-to-one relationship between even perfect numbers and Mersenne primes. It is unknown whether there are any odd perfect numbers.
Yes. The next perfect numbers are 496 and 8128.