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A291503
a(n) is the smallest k such that sigma(k) = phi(n), or 0 if no such k exists.
1
1, 1, 0, 0, 3, 0, 5, 3, 5, 3, 0, 3, 6, 5, 7, 7, 0, 5, 10, 7, 6, 0, 0, 7, 19, 6, 10, 6, 12, 7, 29, 0, 19, 0, 14, 6, 22, 10, 14, 0, 27, 6, 20, 19, 14, 0, 0, 0, 20, 19, 21, 14, 0, 10, 27, 14, 22, 12, 0, 0, 24, 29, 22, 21, 33, 19, 0, 21, 43, 14, 0, 14, 30, 22, 27, 22, 24, 14, 45
OFFSET
1,5
LINKS
FORMULA
a(A069825(n)) = 0 for n > 1.
a(n) = A051444(A000010(n)). - Michel Marcus, Aug 25 2017
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