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 A257865 Smallest k such that phi(k) = n*phi(k+1), where phi(n) = A000010(n) gives the value of Euler's totient function at n. 1
 1, 5, 119, 629, 17907119 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS From Manfred Scheucher, May 27 2015: (Start) a(6)>=3*10^8 (calculation) a(7)>=3.5*10^13, a(8)>=4.5*10^25, a(9)>=3.0*10^47, and so on... (doubly exponential lower bound, see uploaded pdf) 239719159679 and 239742643139 admit a ratio of 5.998... and 6.008..., resp. There might be a relation to the sequence A098026. (End) LINKS Manfred Scheucher, A lower bound on A257865(n) FORMULA a(n) >= exp(exp(c(n-3))) with c=exp(gamma) and gamma being the Euler-Mascheroni_constant (see pdf). - Manfred Scheucher, May 27 2015 EXAMPLE a(3) = 119, because phi(119) == 3*phi(120) = 96 and 119 is the smallest k where this equality holds for n = 3. MATHEMATICA Table[k = 1; While[EulerPhi[k] != n EulerPhi[k + 1], k++]; k, {n, 4}] (* Michael De Vlieger, May 12 2015 *) PROG (PARI) a(n) = my(k=1); while(eulerphi(k)!=n*eulerphi(k+1), k++); k CROSSREFS Cf. A001274, A171262, A256907, A256937, A257550. Sequence in context: A265977 A208387 A144998 * A212040 A067359 A156962 Adjacent sequences:  A257862 A257863 A257864 * A257866 A257867 A257868 KEYWORD nonn,more AUTHOR Felix Fröhlich, May 11 2015 STATUS approved

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Last modified June 24 22:08 EDT 2019. Contains 324337 sequences. (Running on oeis4.)