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 A293184 Numbers n such that bphi(n) = bphi(n+1), where bphi(n) is the bi-unitary analog of Euler's totient function (A116550). 2
 1, 14, 20, 57, 187, 188, 916, 1603, 93928, 142891, 432976 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS 187 is the first solution to bphi(n) = bphi(n+1) = bphi(n+2). LINKS EXAMPLE 14 is in the sequence since bphi(14) = bphi(15) = 9. MATHEMATICA bphi[1] = 1; bphi[n_] := With[{pp = Power @@@ FactorInteger[n]},   Count[Range[n], m_ /; Intersection[pp, Power @@@ FactorInteger[m]] == {}]]; a={}; b1=0; Do[b2 = bphi[k]; If[b1 == b2, a = AppendTo[a, k - 1]]; b1 = b2, {k, 1, 10^3}]; a (* after Jean-François Alcover at A116550 *) PROG (PARI) udivs(n) = {my(d = divisors(n)); select(x->(gcd(x, n/x)==1), d); } gcud(n, m) = vecmax(setintersect(udivs(n), udivs(m))); biuphi(n) = if (n==1, 1, sum(k=1, n-1, gcud(n, k) == 1)); isok(n) = biuphi(n) == biuphi(n+1); lista(nn) = {x = biuphi(1); for (n=2, nn, y = biuphi(n); if (x==y, print1(n-1, ", ")); x = y; ); } \\ Michel Marcus, Nov 09 2017 CROSSREFS Cf. A001274, A116550, A287055. Sequence in context: A259536 A067796 A015797 * A064974 A167335 A129244 Adjacent sequences:  A293181 A293182 A293183 * A293185 A293186 A293187 KEYWORD nonn,more AUTHOR Amiram Eldar, Oct 01 2017 EXTENSIONS a(10) from Michel Marcus, Nov 11 2017 a(11) from Michel Marcus, Nov 12 2017 STATUS approved

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Last modified October 15 05:43 EDT 2019. Contains 328026 sequences. (Running on oeis4.)