

A083249


Numbers n with A045763(n) = n + 1  d(n)  phi(n) < d(n) < phi(n).


5



5, 7, 9, 11, 13, 16, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271
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OFFSET

1,1


COMMENTS

For primes this means 0 < 2 < p1 so primes p greater than 3 are members.
Only two composite solutions below 10000000: n = 9 and n = 16.
From Charles R Greathouse IV, Apr 12 2010: (Start)
d(n) < phi(n) is true for all n > 30 (see A020490), so the main condition is n + 1  d(n)  phi(n) < d(n). Rewrite this as n  phi(n) < 2d(n)  1.
If n is composite, then the cototient n  phi(n) >= sqrt(n).
For n > 32760, d(n) < sqrt(n)/2.
So all composite solutions are in 1..32760. Checking these (and applying the other inequality), the only composite members are 9 and 16.
Thus the sequence is the primes greater than 3, together with 9 and 16.
(End)


LINKS

Michael De Vlieger, Table of n, a(n) for n = 1..10000


EXAMPLE

n = 9 is a member: 3 divisors, 6 coprimes, 1 (it is 6) unrelated: 6 > 3 > 1;
n = 16 is a member: 5 divisors, 8 coprimes 4 unrelateds ({6, 10, 12, 14}): 8 > 5 > 4.


MATHEMATICA

Do[r=EulerPhi[n]; d=DivisorSigma[0, n]; u=nrd+1; If[Greater[r, d]&&Greater[d, u]&&!PrimeQ[n], Print[n, {d, r, u}]], {n, 1, 1000}] (* for composite solutions *) (* corrected by Charles R Greathouse IV, Apr 12 2010 *)
(* Second program: *)
Select[Range@ 272, Function[n, n  (#1 + #2  1) < #1 < #2 & @@ {DivisorSigma[0, n], EulerPhi[n]}]] (* Michael De Vlieger, Jul 22 2017 *)


PROG

(PARI) a(n) = if(n>6, prime(n), [5, 7, 9, 11, 13, 16][n]) \\ Charles R Greathouse IV, Apr 12 2010


CROSSREFS

Cf. A000005, A000010, A045763, A073757, A051953.
Sequence in context: A279914 A123910 A024886 * A077153 A031102 A005763
Adjacent sequences: A083246 A083247 A083248 * A083250 A083251 A083252


KEYWORD

nonn,easy


AUTHOR

Labos Elemer, May 07 2003


EXTENSIONS

Extension, new definition, and edits from Charles R Greathouse IV, Apr 12 2010


STATUS

approved



