A050474 Solutions to 2*phi(x) = x+1.

1, 3, 15, 255, 65535, 83623935, 4294967295, 6992962672132095

Offset

1,2

Comments

If n is in the sequence and n+2 is prime then m=n*(n+2) is in the sequence because 2*phi(m) = 2*phi(n*(n+2)) = 2*phi(n)*(n+1) = (n+1)^2 = m+1. We can obtain the terms 3, 15, 255, 65535 & 4294967295 from 1 (the first term) in this way. Also since 83623935 is a term and 83623935+2 is prime 83623935*(83623935+2)=6992962672132095 is in the sequence. So 1 and 83623935 are the only known independent terms and next term of this sequence if it exists is the third such term. - Farideh Firoozbakht, May 01 2007

The next term, if it exists, has at least 7 distinct prime factors (see Beiler, p. 92). - Jud McCranie, Dec 13 2012

From Chris Boyd, Mar 22 2015: (Start)

Solutions to k*phi(x) = x + 1, including a(1) - a(8), were published in 1932 by D. H. Lehmer. In the paper's summing up, "3*5*353*929" (= 4919055) was printed in error; it should have read "3*5*17*353*929" (= 83623935), i.e., a(6). This error has been propagated in several subsequent texts, including Wong's thesis.

Lehmer identified solutions where x has fewer than 7 distinct prime factors. Wong showed that no additional solutions exist unless x has at least 8 distinct prime factors. It appears not to be excluded by either author that an unidentified solution < a(8) with 8 or more distinct prime factors may exist. (End)

There are no other terms below 10^17. - Max Alekseyev, Oct 30 2021

References

A. H. Beiler, Recreations in the Theory of Numbers, page 92.

Links

Table of n, a(n) for n=1..8.

D. H. Lehmer, On Euler's totient function, Bulletin of the American Mathematical Society, 38 (1932), 745-751.

E. Wong, Computations on normal families of primes, Simon Fraser University, 1997, MSc thesis.

Formula

A number n is in the sequence iff phi(n^2)=1+2+3+...+n because n is in the sequence <=> 2*phi(n)=n+1 <=> n*phi(n)=n*(n+1)/2 <=> phi(n^2)=1+2+3++...+n. For n=1,2,...,5, a(n)=2^2^(n-1)-1. - Farideh Firoozbakht, Jan 26 2006

Example

2*phi(15) = 2*8 = 15 + 1, so 15 is a member of the sequence.

Mathematica

Select[Range[700000], (# + 1)== 2 EulerPhi[#] &] (* Vincenzo Librandi, Mar 22 2015 *)

Prog

(PARI) is_A050474(n)=if(2*eulerphi(n)==n+1, 1, 0) \\ Chris Boyd, Mar 22 2015
(Magma) [n: n in [1..2*10^6] | 2*EulerPhi(n) eq (n+1)]; // Vincenzo Librandi, Mar 22 2015

Crossrefs

Cf. A000010, A129613, A129614, A129615, A202855, A203966.

Sequence in context: A116518 A247174 A277626 * A051179 A122591 A326263

Adjacent sequences: A050471 A050472 A050473 * A050475 A050476 A050477

Keyword

hard,nonn

Author

Jud McCranie, Dec 24 1999

Status

approved