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A182972
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Numerators of positive rationals < 1 arranged by increasing sum of numerator and denominator then by increasing numerator.
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15
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1, 1, 1, 2, 1, 1, 2, 3, 1, 3, 1, 2, 4, 1, 3, 1, 2, 3, 4, 5, 1, 5, 1, 2, 3, 4, 5, 6, 1, 3, 5, 1, 2, 4, 7, 1, 3, 5, 7, 1, 2, 3, 4, 5, 6, 7, 8, 1, 5, 7, 1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 3, 7, 9, 1, 2, 4, 5, 8, 10, 1, 3, 5, 7, 9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 1, 5, 7, 11, 1, 2, 3, 4, 6, 7, 8, 9, 11, 12, 1, 3, 5, 7, 9, 11
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,4
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COMMENTS
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REFERENCES
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S. Cook, Problem 511: An Enumeration Problem, Journal of Recreational Mathematics, Vol. 9:2 (1976-77), 137. Solution by the Problem Editor, JRM, Vol. 10:2 (1977-78), 122-123.
R. K. Guy, Unsolved Problems in Number Theory (UPINT), Section D11.
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LINKS
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EXAMPLE
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Positive fractions < 1 listed by increasing sum of numerator and denominator, and by increasing numerator for equal sums:
1/2
1/3
1/4 2/3
1/5
1/6 2/5 3/4
1/7 3/5
1/8 2/7 4/5
1/9 3/7
1/10 2/9 3/8 4/7 5/6
1/11 5/7
1/12 2/11 3/10 4/9 5/8 6/7
1/13 3/11 5/9
1/14 2/13 4/11 7/8
1/15 3/13 5/11 7/9
1/16 2/15 3/14 4/13 5/12 6/11 7/10 8/9
1/17 5/13 7/11
1/18 2/17 3/16 4/15 5/14 6/13 7/12 8/11 9/10
1/19 3/17 7/13 9/11
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MAPLE
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t1:=[];
for n from 2 to 40 do
t1:=[op(t1), 1/(n-1)];
for i from 2 to floor((n-1)/2) do
if gcd(i, n-i)=1 then t1:=[op(t1), i/(n-i)]; fi; od:
od:
t1;
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MATHEMATICA
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t1={}; For[n=2, n <= 40, n++, AppendTo[t1, 1/(n-1)]; For[i=2, i <= Floor[(n-1)/2], i++, If[GCD[i, n-i] == 1, AppendTo[t1, i/(n-i)]]]]; t1 // Numerator // Rest (* Jean-François Alcover, Jan 20 2015, translated from Maple *)
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PROG
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(Pascal) program a182972;
var
num, den, n: longint;
function gcd(i, j: longint):longint;
begin
repeat
if i>j then i:=i mod j else j:=j mod i;
until (i=0) or (j=0);
if i=0 then gcd:=j else gcd:=i;
end;
begin
num:=1; den:=1; n:=0;
repeat
repeat
inc(num); dec(den);
if num>=den then
begin
inc(den, num); num:=1;
end;
until gcd(num, den)=1;
inc(n); writeln(n, ' ', num);
until n=100000;
end.
(Haskell)
a182972 n = a182972_list !! (n-1)
a182972_list = map fst $ concatMap q [3..] where
q x = [(num, den) | num <- [1 .. div x 2],
let den = x - num, gcd num den == 1]
(Python)
from itertools import count, islice
from math import gcd
def A182972_gen(): # generator of terms
return (i for n in count(2) for i in range(1, 1+(n-1>>1)) if gcd(i, n-i)==1)
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CROSSREFS
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KEYWORD
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nonn,easy,frac,nice
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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