OFFSET
1,5
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
FORMULA
a(A069825(n)) = 0 for n > 1.
EXAMPLE
a(5) = 3 because sigma(3) = phi(5) and 3 is the smallest number with this property.
MAPLE
N:= 100: # to get a(1)..a(N)
R:= Vector(N):
for k from 1 to N-1 do
s:= numtheory:-sigma(k);
if s <= N and R[s] = 0 then R[s]:= k fi;
od:
seq(R[numtheory:-phi(n)], n=1..N); # Robert Israel, Aug 25 2017
PROG
(PARI) a(n) = for(k=1, eulerphi(n), if(sigma(k)==eulerphi(n), return(k))); 0 \\ after Charles R Greathouse IV at A051444
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Altug Alkan, Aug 25 2017
STATUS
approved