%I
%S 2,1,0,4,4,4,14,6,6,4,16,6,14,6,0,5,8,6,6,8,0,4,46,12,10,8,6,12,26,12,
%T 62,6,12,4,16,12,28,6,0,10,24,24,86,8,0,6,38,6,62,25,12,16,24,18,32,
%U 24,0,4,118,24,80,6,12,10,28,12,134,8,0,35,142,24,146,8,30,12,8,24,46,20,6
%N Conjectured values for a(n) = least natural number k such that phi(n+k) = phi(n) + phi(k), if k exists; otherwise 0.
%C It would be nice to remove the word "Conjectured" from the description.  _N. J. A. Sloane_
%C The values of a(3), a(15) and a(21) listed above, namely 0, are conjectural. There is no natural number k < 10^6 satisfying the "homomorphic condition" phi(n+k) = phi(n) + phi(k) for n = 3, 5, 21.
%C The terms for which there is no solution k < 10^6 are n = 3, 15, 21, 39, 45, 57, 69, 105, 147, 165, 177, 195, 213, 273, 285,..., which satisfy n=3 (mod 6).  _T. D. Noe_, Jan 20 2004
%C All n < 2000 and k < 10^8 have been tested. Sequence A110172 gives the n for which there is no solution k < 10^8. For n=1 (mod 3) or n=2 (mod 3), it appears that the least solution k satisfies k<=2n. For n=0 (mod 3), the least k, if it exists, can be greater than 2n.  _T. D. Noe_, Jul 15 2005
%D R. K. Guy, Unsolved Problems in Number Theory, 3rd Ed., New York, SpringerVerlag, 2004, Section B36.
%t a[ n_ ] := Min[ Select[ Range[ 1, 10^6 ], EulerPhi[ 1, n + # ] == EulerPhi[ 1, n ] + EulerPhi[ 1, # ] & ] ]; Table[ a[ i ], {i, 1, 21} ]
%Y Cf. A000010.
%Y Cf. A091531 (primes p such that k=2p is the smallest solution to phi(p+k) = phi(p) + phi(k)).
%Y Cf. A110173 (least k such that phi(n) = phi(k) + phi(nk) for 0 < k < n).
%K nonn
%O 1,1
%A _Joseph L. Pe_, Dec 27 2001
%E More terms from _T. D. Noe_, Jan 20 2004
