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Thursday, 10 October 2019

Totient and Sigma <--> Primes and Biprimes

Though it was obvious when I looked into it, I only recently realised that, for any prime number, the average of its totient and sum of divisors is equal to the number itself. This is because for a prime number p, it's only divisors are itself and 1, thus the sum of divisors is p+1. For the totient, the only number that is coprime with p is 1 and thus the totient is p1. Adding the sum of divisors and the totient together gives 2p and the average is p. Thus formally stated:


In a similar vein, it was only today that I discovered that for a number that is biprime, sometimes called semiprime, the average of its sum of divisors and totient is always one more than the number. Again, this was obvious once I looked into it. The reason is that a biprime number n has only two factors, let's say a and b. Thus n=ab. The sum of divisors is thus n+a+b+1. For the totient, there are b1 multiples of a and a1 multiples of b that are coprime with n. Thus the totient is nab1, remembering that 1 is regarded as being coprime as well. Adding the sum of divisors and the totient together and averaging, we get:(n+a+b+1)(nab1)2=2n+22=n+1
This is all very simple stuff but I'd never seen this connection between the totients and sums of divisors of primes and biprimes formally stated before. Thus formally stated:


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