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%I #18 Sep 20 2018 08:16:46
%S 1,2,5,21,9177,244965,14307854,24571869,128199057,140830365,401767629,
%T 420567854,468190437,525970977,780768165,886434645,1597167645,
%U 4046753949,4473784821,5364666165,5515718205,11175736334,14408460165,18026319710,20106993885,20357733129
%N Numbers n such that uphi(n) = uphi(n+2) - uphi(n+1), where uphi is the unitary totient function (A047994).
%e 21 is in the sequence since uphi(21) = 12 = uphi(23) - uphi(22) = 22 - 10.
%t uphi[n_] := If[n>1,(Times @@ (Table[ #[[1]]^ #[[2]] - 1, {1} ] & /@ FactorInteger[n]))[[1]],1]; Select[Range[10^7], uphi[#] == uphi[# + 2] - uphi[# + 1] &]
%o (PARI) uphi(n) = my(f=factor(n)); prod(i=1, #f~, f[i, 1]^f[1, 2]-1);
%o isok(n) = uphi(n) == uphi(n+2) - uphi(n+1); \\ _Michel Marcus_, Jan 26 2018
%Y Cf. A047994, A229552 (corresponding sequence with phi).
%K nonn
%O 1,2
%A _Amiram Eldar_, Jan 26 2018
%E a(7)-a(26) from _Giovanni Resta_, Jan 26 2018
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