OFFSET
1,2
COMMENTS
For each n there are infinitely many numbers k for which n divides sigma(k) and phi(k). - Marius A. Burtea, Mar 28 2019
LINKS
Giovanni Resta, Table of n, a(n) for n = 1..10000 (terms 1-388 from Marius A. Burtea, 389-2808 from David A. Corneth)
EXAMPLE
MATHEMATICA
Array[Block[{i = 1}, While[Mod[GCD[DivisorSigma[1, i], EulerPhi@ i], #] != 0, i++]; i] &, 50] (* Michael De Vlieger, Mar 28 2019 *)
PROG
(PARI) a(n)={my(k=1); while(gcd(sigma(k), eulerphi(k))%n!=0, k++); k}
(Magma) [Min([n: n in [1..300000] | IsIntegral(SumOfDivisors(n)/m) and IsIntegral(EulerPhi(n)/m) ]): m in [1..70]]; // Marius A. Burtea, Mar 28 2019
(Magma) v:=[];
for n in [1..60] do
m:=1;
while not EulerPhi(m) mod n eq 0 or not SumOfDivisors(m) mod n eq 0 do
v[n]:=0;
m:=m+1;
end while;
v[n]:=m;
end for;
v; // Marius A. Burtea, Mar 30 2019
CROSSREFS
KEYWORD
nonn,changed
AUTHOR
Phil Carmody, Mar 01 2013
STATUS
approved