OFFSET
1,1
COMMENTS
Suppose that x > 0. A fraction p/q of primes is a lower primes-only best approximate, and we write "p/q is in L(x)", if u/v < p/q < x < p'/q for all primes u and v such that v < q, where p' is least prime > p.
Let q(1) be the least prime q such that u/q < x for some prime u, and let p(1) be the greatest such u. The sequence L(x) follows inductively: for n > 1, let q(n) is the least prime q such that p(n)/q(n) < p/q < x for some prime p. Let q(n+1) = q and let p(n+1) be the greatest prime p such that p(n)/q(n) < p/q < x.
For a guide to POBAs, lower POBAs, and upper POBAs, see A265759.
EXAMPLE
The lower POBAs to sqrt(5) start with 3/2, 5/3, 11/5, 29/13, 163/73, 199/89, 521/233. For example, if p and q are primes and q > 73, and p/q < sqrt(5), then 163/73 is closer to sqrt(5) than p/q is.
MATHEMATICA
x = Sqrt[5]; 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] (* A265784 *)
Denominator[tL] (* A265785 *)
Numerator[tU] (* A265786 *)
Denominator[tU] (* A265787 *)
Numerator[y] (* A222588 *)
Denominator[y] (* A265789 *)
CROSSREFS
KEYWORD
nonn,frac,more
AUTHOR
Clark Kimberling, Dec 23 2015
EXTENSIONS
a(9)-a(10) from Robert Price, Apr 05 2019
STATUS
approved