OFFSET
1,2
LINKS
Robert Israel, Table of n, a(n) for n = 1..1000
MAPLE
with(numtheory): A069825 := proc(n) local p, i; p := phi(n); for i from 1 to n-1 do if sigma(i) = p then RETURN(NULL) fi od; n end; # Peter Luschny, Nov 02 2010
# Alternative:
N:= 1000: # to get terms <= 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:
1, op(select(t -> R[numtheory:-phi(t)]=0, [$2..N])); # Robert Israel, Aug 25 2017
MATHEMATICA
okQ[n_] := NoneTrue[Range[n-1], DivisorSigma[1, #] == EulerPhi[n]&];
Select[Range[250], okQ] (* Jean-François Alcover, Jun 03 2019 *)
PROG
(PARI) for(n=1, 600, if(abs(prod(i=1, n-1, eulerphi(n)-sigma(i)))>0, print1(n, ", ")))
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Benoit Cloitre, Apr 28 2002
STATUS
approved