A204540 -id:A204540 - cp's OEIS frontend

A204539 a(n) is the number of integers N=4k whose "basin" sequence (cf. comment) ends in n^2.

1, 1, 1, 2, 1, 3, 2, 4, 2, 4, 3, 5, 1, 9, 2, 10, 3, 5, 7, 9, 2, 10, 9, 9, 2, 13, 9, 8, 4, 20, 4, 15, 6, 15, 8, 12, 6, 22, 6, 15, 15, 21, 5, 13, 12, 23, 7, 24, 11, 19, 15, 24, 6, 30, 6, 26, 7, 27, 26, 13, 6, 33, 27, 30, 5, 13, 30, 30, 5, 37, 15, 26, 28, 32, 7, 17, 25, 54, 9, 30, 21, 41, 25

Offset: 2

Colm Fagan, Jan 16 2012

nonn

The "basin" (analogous to river basins, for reasons set out below) is the number of positive integers N=4k which end in the "sea" at n^2. The "sea" of N is found as follows:
Starting out with N, in step i=1,2,3,..., stop if you have reached N=(i+1)^2 (the "sea" of N), otherwise set N to the next higher, odd or even (according to the parity of i), multiple of i+2, and go to step i+1.
Partial sums of this sequence appear to be A104738 (with a shift in offset). This has been confirmed for at least the first 4000 terms, but it is not at all clear why this is the case. - Ray Chandler, Jan 20 2012
After the first term, this sequence agrees with A028914 except for offset. Therefore this sequence is related to A028913, A007952, A002491 and A108696 dealing with the sieve of Tchoukaillon (or Mancala, or Kalahari). - Ray Chandler, Jan 20 2012

			For integers N=4,8,12,16,... we have the following sequences:
  {4}
  {8, 9}   (8 -> the next higher odd multiple of 3, which is 9 -> STOP)
  {12, 15, 16}  (12 -> 3*5=15 -> 4*4=16 -> STOP)
  {16, 21, 24, 25}
  {20, 21, 24, 25}
  {24, 27, 32, 35, 36}
  {28, 33, 40, 45, 48, 49}
  {32, 33, 40, 45, 48, 49}
  {36, 39, 40, 45, 48, 49}
  ...
Thus there is 1 integer N=4k ending in the sea at 2^2, whence basin a(2)=1, and idem for 3 and 4.
The two integers 16 and 20 end at 5^2, so the basin of 5 is a(5)=2.
There is again a(6)=1 integer ending in 6^2, while the basin of 7 are the 3 integers 28, 32, and 36, which all merge into the "river" that enters the "sea" in 7^2=49.
Thus the first 6 terms in the sequence are 1, 1, 1, 2, 1, 3.
Take N=100 as an example: the next integer on the same line is the next higher odd multiple of 3, i.e., smallest 3*(2m+1) > 100, which is 105. The next number is the least even multiple of 4, 4*(2m) = 112, etc., leading to 115 = 5*(2m+1), followed by 120 = 6*(2m), 133 = 7*(2m+1), 144 = 8*2m (where we have a square, but not the square of 8), 153 =9*(2m+1), 160 = 10*2m, 165 = 11*(2m+1), 168 = 12*(2m) and finally 169 = 13*13.

Ray Chandler, Table of n, a(n) for n = 2..10001
Mark Dukes, Fagan's Construction, Strange Roots, and Tchoukaillon Solitaire, Journal of Integer Sequences, Vol. 24 (2021), Article 21.7.1.
Mark Dukes, Fagan's Construction, Strange Roots, and Tchoukaillon Solitaire, arXiv:2202.02381 [math.NT], 2022.

Cf. A002491, A007952, A028913, A104738, A108696, A204540.

Essentially the same as A028914.

cumul[n_Integer] := Module[{den1 = n, num = n^2, den2}, While[num > 4 && den1 != 2, num = num - 1; den1 = den1 - 1; den2 = Floor[num/den1]; If[Not[EvenQ[den1 + den2]], den2 = den2 - 1]; num = den1 den2]; Return[num/4]]; basin[2] := 1; basin[n_Integer] := cumul[n] - cumul[n - 1]; Table[basin[n], {n, 2, 75}] (* Alonso del Arte, Jan 19 2012 *)

bs(n,s,m=2)={while(n>m^2,n=(n\m+++2-bittest(n\m-m,0))*m; s & print1(n","));n}
n=4; for(c=2,50, for(k=1,9e9, bs(n+=4)==c^2 || print1(k",")||break)) \\ M. F. Hasler, Jan 20 2012

A185001 Numbers k with the property that their basins (as defined in A204539) are 2.

Original entry on oeis.org

5, 8, 10, 16, 22, 26, 106, 110, 182, 234, 282, 288, 318, 434, 766, 1056, 1072, 1462, 1550, 1930, 3024, 4330, 5424, 9398, 10634, 53094, 90602, 151632, 384002, 511638, 530102, 1364850, 1887006, 2193072, 3138096, 6470672, 6959070

Offset: 1

Colm Fagan, Jan 23 2012

This sequence follows on from A204540, which lists seven values of k for which basin(k) = 1. There are 37 known values of k for which basin(k) = 2. A search of numbers up to 10,000,000 has not uncovered any further integers with this property. The possibility that there are even larger numbers with basins equal to 2 cannot be completely ruled out, but the chances of one being discovered are remote, in view of the fact that the average basin size for large values of k is approximately k/3, that is, over 2000000 in the region where the last known such integer was discovered.

For unknown reasons, all integers > 5 with basins equal to 1 or 2 are even.

Mark Dukes, Fagan's Construction, Strange Roots, and Tchoukaillon Solitaire, Journal of Integer Sequences, Vol. 24 (2021), Article 21.7.1.
Mark Dukes, Fagan's Construction, Strange Roots, and Tchoukaillon Solitaire, arXiv:2202.02381 [math.NT], 2022.

Cf. A204539, A204540.

cp's OEIS Frontend

A204539 a(n) is the number of integers N=4k whose "basin" sequence (cf. comment) ends in n^2.

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