A257550
Numbers n such that phi(n) = 5*phi(n+1).
Original entry on oeis.org
17907119, 18828809, 31692569, 73421039, 179467469, 322757819, 337567229, 627702389, 975314339, 2537636009, 2722271369, 3328653509, 3917646809, 5529412349, 6369847469, 11179199849, 11201693579, 11363832479, 13442120999, 16781760449, 19751331599, 20002320029
Offset: 1
phi(17907119) = 16588800 = 5*phi(17907120).
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.
Original entry on oeis.org
1, 5, 119, 629, 17907119
Offset: 1
a(3) = 119, because phi(119) == 3*phi(120) = 96 and 119 is the smallest k where this equality holds for n = 3.
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Table[k = 1; While[EulerPhi[k] != n EulerPhi[k + 1], k++]; k, {n, 4}] (* Michael De Vlieger, May 12 2015 *)
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a(n) = my(k=1); while(eulerphi(k)!=n*eulerphi(k+1), k++); k
A268126
Numbers n such that phi(n) = 4*phi(n-1).
Original entry on oeis.org
1261, 13651, 17557, 18721, 24511, 42121, 113611, 244531, 266071, 712081, 749911, 795691, 992251, 1080721, 1286731, 1458271, 1849471, 2271061, 2457691, 3295381, 3370771, 3414841, 3714751, 4061971, 4736491, 5314051, 5827081, 6566911, 6935083, 7303981, 7864081
Offset: 1
1261 is in the sequence because phi(1261) = 1152 = 4*phi(1260) = 4*288.
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[n: n in [2..10^7] | EulerPhi(n) eq 4*EulerPhi(n-1)]
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Select[Range@10000000, EulerPhi@# == 4 EulerPhi[# - 1] &] (* Vincenzo Librandi, Jan 27 2016 *)
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isok(n) = (eulerphi(n) == 4*eulerphi(n-1)); \\ Michel Marcus, Jan 27 2016
Showing 1-3 of 3 results.
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