OFFSET
1,2
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
W. D. Banks, J. B. Friedlander, C. Pomerance, and I. E. Shparlinski, Multiplicative structure of values of the Euler function, in High primes and misdemeanours: Lectures in honour of the sixtieth birthday of Hugh Cowie Williams, Fields Inst. Comm. 41 (2004), pp. 29-47.
Paul Pollack and Carl Pomerance, Square values of Euler's function, Bulletin of the London Mathematical Society 46:2 (April 2014), pp. 403-414.
FORMULA
a(n) = sqrt(A221285(n)).
Pollack and Pomerance show that n (log n)^0.0063 << a(n) << n (log n)^3.
MATHEMATICA
inversePhiSingle[(m_)?EvenQ] := Module[{p, nmax, n}, p = Select[Divisors[m] + 1, PrimeQ]; nmax = m*Times @@ (p/(p-1)); n = m; While[n <= nmax, If[EulerPhi[n] == m, Return[n]]; n++]; 0];
Reap[For[k = 1, k <= 200, k = k + If[k==1, 1, 2], If[inversePhiSingle[k^2] > 0, Print[k]; Sow[k]]]][[2, 1]] (* Jean-François Alcover, Dec 11 2018 *)
PROG
(PARI) is(n)=istotient(n^2)
CROSSREFS
KEYWORD
nonn
AUTHOR
Charles R Greathouse IV, Feb 05 2013
STATUS
approved