a number is said to be a perfect number when the sum of all its divisors is twice the number itself. find the sum of all PERFECT NUMBERS of the form +1
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let us consider the only two cases possible one by one let N= n n +1 CASE 1:- let n be odd.since N is an even perfect number,N must of the form N= 2 m − 1 ( 2 m -1),(by euclid's theorem),where 2 m − 1 is a prime. clearly,N can be factored as N = n n +1 = (n+1)r,where r= n n − 1 - n n − 2 + ... -n+1.we now claim that (n+1,r)=1.to show this,notice that since n is odd ,r is odd and n+1 is even.let n+1= 2 s t,where t is an odd positive integer.then N= 2 s tr,where both t and r are odd.since N is an even perfect number,this possible only if t=1;so n+1= 2 s and hence (n+1,r)=1(notice that if r=1,then N= n n +1=n+1;so n=1.then N=2 which is not a perfect number) Since N= 2 m − 1 ( 2 m -1)=(n+1)r= 2 s r ,where 2 m − 1 is a prime and r is odd, 2 s = 2 m − 1 = n + 1 and r= ( 2 m -1)=2. 2 m − 1 -1 = 2(n+1)-1=2n+1 Therefore, N= n n +1= ( n + 1 ) ( 2 n + 1 ) = 2 n 2 +3n+1 this yields n n = 2 n 2 +3n
n n − 1 = 2 n + 3 since n is an integer this equation has an unique solution 3 it can be verified algebraically. then N= n n +1=28 thus 28 is the only even perfect number!
further, i claim that there are no odd perfect numbers of the form n n +1. thus our answer is 28. if anyone can furnish us with a proof of the non existence of an odd perfect number of the mentioned form it will delightful if not,i will post it at a future date