OFFSET
1,1
COMMENTS
Also numbers n such that n+1-phi(n) | phi(n).
A203966 lists the numbers n such that the sum of numbers x<=n coprime to n divides the sum of numbers y<=n not coprime to n. This is equivalent to numbers n such that phi(n) | n+1. [suggested by Giovanni Resta]
LINKS
Paolo P. Lava, Table of n, a(n) for n = 1..1000
EXAMPLE
The numbers coprime to 15 are 1, 2, 4, 7, 8, 11, 13, 14 and their sum is 60. In fact 15*phi(15)/2 = 60.
The sum of the numbers from 1 to 15 is 15*(15+1)/2 = 120 and therefore the sum of the numbers not coprime to 15 is 120 - 60 = 60. At the end we have that 60/60 = 1.
MAPLE
with(numtheory); P:=proc(q) local i, n;
for n from 2 to q do if not isprime(n) then
if type(phi(n)/(n+1-phi(n)), integer) then print(n);
fi; fi; od; end: P(10^6);
CROSSREFS
KEYWORD
nonn
AUTHOR
Paolo P. Lava, Feb 21 2014
STATUS
approved