The OEIS mourns the passing of Jim Simons and is grateful to the Simons Foundation for its support of research in many branches of science, including the OEIS.
The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A279621 Numbers k such that 1/phi(x) + 1/phi(y) = 1/phi(k), for some x + y = k and phi(k) is the Euler totient function of k. 1
 1890, 2100, 2310, 3780, 5250, 7770, 10080, 11310, 11550, 11880, 12180, 13230, 13650, 13860, 14190, 14910, 15750, 17640, 18060, 19950, 20460, 20790, 21630, 22050, 22110, 23100, 24090, 24180, 24570, 25410, 25620, 25830, 26070, 27090, 27510, 27720, 28980, 29040, 29400 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All terms appear to be multiples of 30. Terms that are not divisible by 30: 70224, 72072, 96558, 114114, 122892, 156156, 166782, 184338, 191268, ... - Amiram Eldar, Jul 22 2019 LINKS Amiram Eldar, Table of n, a(n) for n = 1..1000 EXAMPLE 1890 = 817 + 1073 and 1/phi(817) + 1/phi(1073) = 1/756 + 1/1008 = 1/432 = 1/phi(1890). The first term with more than one solution is 14190: 14190 = 6319 + 7871 and 1/phi(6319) + 1/phi(7871) = 1/6160 + 1/7392 = 1/3360 = 1/phi(14190). 14190 = 6443 + 7747 and 1/phi(6443) + 1/phi(7747) = 1/6048 + 1/7560 = 1/3360 = 1/phi(14190). MAPLE with(numtheory): P:= proc(q) local k, n; for n from 1 to q do for k from 1 to trunc(n/2) do if 1/phi(k)+1/phi(n-k)=1/phi(n) then print(n); break; fi; od; od; end: P(10^6); MATHEMATICA aQ[n_] := Module[{k = 1, r = 1/EulerPhi[n]}, While[2*k <= n && 1/EulerPhi[k] + 1/EulerPhi[n - k] != r, k++]; 2*k <= n]; (* Amiram Eldar, Jul 22 2019 *) CROSSREFS Cf. A000010. Sequence in context: A106764 A157486 A086476 * A168226 A151721 A187743 Adjacent sequences: A279618 A279619 A279620 * A279622 A279623 A279624 KEYWORD nonn AUTHOR Paolo P. Lava, Dec 19 2016 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified June 19 00:13 EDT 2024. Contains 373491 sequences. (Running on oeis4.)