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%I #19 Mar 26 2020 07:17:01
%S 0,0,1,1,1,1,1,1,2,2,1,2,1,2,3,1,1,2,1,2,3,2,1,2,2,2,2,2,1,3,1,1,3,2,
%T 3,2,1,2,3,2,1,3,1,2,3,2,1,2,2,2,3,2,1,2,3,2,3,2,1,3,1,2,3,1,3,3,1,2,
%U 3,3,1,2,1,2,3,2,3,3,1,2,2,2,1,3,3,2,3,2,1,3,3,2,3,2,3,2,1,2,3,2,1,3,1,2,5
%N Number of distinct terms in the first difference sequence of reduced residue system[=dRRS] for n.
%H Michael De Vlieger, <a href="/A061498/b061498.txt">Table of n, a(n) for n = 1..10000</a>
%e For n=24, RRS={1, 5, 7, 11, 13, 17, 19, 23}, dRRS={4, 2, 4, 2, 4, 2, 4};
%e for n=32, dRRS={2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2};
%e for n=31, dRRS={1, 1, 1, 1, 1, 1, 1, 1, 1, 1, ...};
%e for n=34, RRS={1, 3, 5, 7, 9, 11, 13, 15, 19, 21, 23, 25, 27, 29, 31, 33}, dRRS={2, 2, 2, 2, 2, 2, 2, 4, 2, 2, 2, 2, 2, 2, 2};
%e so a(24)=2, a(32)=1, a(31)=1, a(34)=2.
%t Table[Length@ Union@ Differences@ Select[Range@ n, CoprimeQ[#, n] &], {n, 105}] (* _Michael De Vlieger_, Jul 15 2017 *)
%o (PARI) a(n) = {my(va = select(x->(gcd(n, x)==1), [1..n])); vd = vector(#va-1, k, va[k+1] - va[k]); #Set(vd);} \\ _Michel Marcus_, Jul 08 2018
%Y Cf. A076362.
%K nonn
%O 1,9
%A _Labos Elemer_, Jun 12 2001
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