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 A261491 a(n) = ceiling(2 + sqrt(8*n-4)). 4
 4, 6, 7, 8, 8, 9, 10, 10, 11, 11, 12, 12, 12, 13, 13, 14, 14, 14, 15, 15, 15, 16, 16, 16, 16, 17, 17, 17, 18, 18, 18, 18, 19, 19, 19, 19, 20, 20, 20, 20, 20, 21, 21, 21, 21, 22, 22, 22, 22, 22, 23, 23, 23, 23, 23, 24, 24, 24, 24, 24, 24, 25, 25, 25, 25, 25, 26, 26, 26, 26, 26, 26, 27, 27, 27, 27, 27, 27, 28, 28, 28, 28, 28, 28, 28, 29 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Conjecture: a(n) = minimal number of stones needed to surround area n in the middle of a Go board (infinite if needed). The formula was constructed this way: when the area is in a diamond shape with x^2+(x-1)^2 places, it can be surrounded by 4x stones. So, a(1)=4, a(5)=8, a(13)=12 etc. The positive solution to the quadratic equation 2x^2 - 2x + 1 = n is x = (2 + sqrt(8n-4))/4. And since a(n)=4x, the formula a(n) = 2 + sqrt(8n-4) holds for the positions mentioned. But incredibly also the intermediate results seem to match when the ceiling function is used. The opposite of this would be an area of 1 X n; it demands the maximal number of stones, a(n) = 2 + 2n. LINKS Kival Ngaokrajang, Illustration of initial terms FORMULA a(n) = ceiling(2 + sqrt(8*n-4)). EXAMPLE Start with the 5-cell area that is occupied by 0's and surrounded by stones 1..8. Add those surrounding stones to the area, one by one. At points 1, 2, 4 and 6, the number of surrounding stones is increased; elsewhere, it is not. Next, do the same with stones A..L. At points A, C, F and I, the number of surrounding stones is increased; elsewhere, it is not. ___D___ __A5C__ _B104E_ G30007J _F206I_ __H8K__ ___L___ MATHEMATICA Array[Ceiling[2 + Sqrt[8 # - 4]] &, {86}] (* Michael De Vlieger, Oct 23 2015 *) PROG (PARI) a(n)=sqrtint(8*n-5)+3 \\ Charles R Greathouse IV, Aug 21 2015 CROSSREFS Cf. A001971. Sequence in context: A300707 A296183 A021876 * A005670 A234948 A123860 Adjacent sequences:  A261488 A261489 A261490 * A261492 A261493 A261494 KEYWORD nonn,easy AUTHOR Juhani Heino, Aug 21 2015 STATUS approved

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Last modified April 6 03:52 EDT 2020. Contains 333267 sequences. (Running on oeis4.)