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A020886 Ordered semiperimeters of primitive Pythagorean triangles. 28
6, 15, 20, 28, 35, 42, 45, 63, 66, 72, 77, 88, 91, 99, 104, 110, 117, 120, 130, 143, 153, 156, 165, 170, 187, 190, 195, 204, 209, 210, 221, 228, 231, 238, 247, 255, 266, 272, 273, 276, 285, 299, 304, 322, 323, 325, 336, 342, 345, 350, 357, 368, 378, 391, 399 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

n is in this sequence iff A078926(n)>0. This is a subset of A005279.

LINKS

Ray Chandler, Table of n, a(n) for n = 1..10000 (duplicates removed by Sean A. Irvine)

FORMULA

a(n) = A024364(n)/2.

MAPLE

isA020886 := proc(an) local r::integer, s::integer ; for r from floor((an/2)^(1/2)) to floor(an^(1/2)) do for s from r-1 to 1 by -2 do if r*(r+s) = an and gcd(r, s) < 2 then RETURN(true) ; fi ; if r*(r+s) < an then break ; fi ; od ; od : RETURN(false) ; end : for n from 2 to 400 do if isA020886(n) then printf("%d, ", n) ; fi ; od ; # R. J. Mathar, Jun 08 2006

MATHEMATICA

A078926[n_] := Sum[Boole[n < d^2 < 2n && CoprimeQ[d, n/d]], {d, Divisors[ n/2^IntegerExponent[n, 2]]}];

Select[Range[1000], A078926[#]>0&] (* Jean-Fran├žois Alcover, Mar 23 2020 *)

CROSSREFS

Cf. A005279, A024364, A078926.

Cf. A020882, A020883, A020884, A020885.

Sequence in context: A294651 A044439 A128253 * A093508 A302022 A094183

Adjacent sequences:  A020883 A020884 A020885 * A020887 A020888 A020889

KEYWORD

nonn

AUTHOR

Clark Kimberling

STATUS

approved

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Last modified August 4 02:10 EDT 2020. Contains 336201 sequences. (Running on oeis4.)