A256946 - OEIS

%I #36 Sep 17 2019 08:57:58

%S 1,2,3,1,4,5,2,6,7,3,8,1,4,9,10,5,11,2,6,12,13,7,3,14,8,15,1,4,9,16,

%T 17,10,5,18,11,19,2,6,12,20,21,13,7,22,3,14,23,8,15,24,1,4,9,16,25,26,

%U 17,10,27,5,18,28,11,19,29,2,6,12,20,30,31,21,13,7,32,22,3,14,33,23,8,34,15,24,35

%N Irregular triangle where n-th row is integers from 1 to n*(n+2), sorted with first squares in order, then remaining numbers by fractional part of the square root.

%C This is a fractal sequence.

%C T(n,k) = T(n+1,A256507(n,k),k), that is, A256507 gives the positions of n-th's row terms in row n+1. - _Reinhard Zumkeller_, Apr 22 2015

%H Franklin T. Adams-Watters, <a href="/A256946/b256946.txt">Table of n, a(n) for n = 1..10385</a> (Rows 1 to 30, flattened.)

%e The table starts:

%e 1 2 3

%e 1 4 5 2 6 7 3 8

%e 1 4 9 10 5 11 2 6 12 13 7 3 14 8 15

%e 1 4 9 16 17 10 5 18 11 19 2 6 12 20 21 13 7 22 3 14 23 8 15 24

%t row[n_] := SortBy[Range[n(n+2)], If[IntegerQ[Sqrt[#]], 0, N[FractionalPart[ Sqrt[#]]]]&];

%t Array[row, 5] // Flatten (* _Jean-François Alcover_, Sep 17 2019 *)

%o (PARI) arow(n)=vecsort(vector(n*(n+2),k,if(issquare(k),0.,sqrt(k)-floor(sqrt(k)))),,1) \\ This relies on vecsort being stable.

%o (Haskell)

%o import Data.List (sortBy); import Data.Function (on)

%o a256946 n k = a256946_tabf !! (n-1) !! (k-1)

%o a256946_row n = a256946_tabf !! (n-1)

%o a256946_tabf = f 0 [] [] where

%o    f k us vs = (xs ++ ys) : f (k+1) xs ys where

%o      xs = us ++ qs

%o      ys = sortBy (compare `on`

%o                   snd . properFraction . sqrt . fromIntegral) (vs ++ rs)

%o      (qs, rs) = span ((== 1) . a010052') [k*(k+2)+1 .. (k+1)*(k+3)]

%o -- _Reinhard Zumkeller_, Apr 22 2015

%Y Cf. A005563 (row lengths and last of each row), A083374 (row sums).

%Y Cf. A256507.

%K nonn,tabf,nice

%O 1,2

%A _Franklin T. Adams-Watters_, Apr 19 2015