OFFSET
1,1
COMMENTS
Suppose that x > 0. A fraction p/q of primes is a primes-only best approximate (POBA), and we write "p/q in B(x)", if 0 < |x - p/q| < |x - u/v| for all primes u and v such that v < q, and also, |x - p/q| < |x - p'/q| for every prime p' except p. Note that for some choices of x, there are values of q for which there are two POBAs. In these cases, the greater is placed first; e.g., B(3) = (7/2, 5/2, 17/5, 13/5, 23/7, 19/7, ...). See A265759 for a guide to related sequences.
Is this related to A165571? - R. J. Mathar, Jan 10 2016
EXAMPLE
The POBAs to tau start with 5/2, 3/2, 5/3, 11/7, 31/19, 37/23, 47/29, 157/97, 571/353, 911/563. For example, if p and q are primes and q > 29, then 47/29 is closer to tau than p/q is.
MATHEMATICA
x = GoldenRatio; z = 1000; p[k_] := p[k] = Prime[k];
t = Table[Max[Table[NextPrime[x*p[k], -1]/p[k], {k, 1, n}]], {n, 1, z}];
d = DeleteDuplicates[t]; tL = Select[d, # > 0 &] (* lower POBA *)
t = Table[Min[Table[NextPrime[x*p[k]]/p[k], {k, 1, n}]], {n, 1, z}];
d = DeleteDuplicates[t]; tU = Select[d, # > 0 &] (* upper POBA *)
v = Sort[Union[tL, tU], Abs[#1 - x] > Abs[#2 - x] &];
b = Denominator[v]; s = Select[Range[Length[b]], b[[#]] == Min[Drop[b, # - 1]] &];
Numerator[tL] (* A265796 *)
Denominator[tL] (* A265797 *)
Numerator[tU] (* A265798 *)
Denominator[tU] (* A265799 *)
Numerator[y] (* A265800 *)
Denominator[y] (* A265801 *)
CROSSREFS
KEYWORD
nonn,frac,more
AUTHOR
Clark Kimberling, Jan 02 2016
EXTENSIONS
a(15)-a(18) from Robert Price, Apr 06 2019
STATUS
approved