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 A233074 Numbers that are exactly midway between the nearest square and the nearest triangular number. 4
 2, 5, 23, 32, 47, 52, 65, 86, 140, 161, 170, 193, 203, 228, 266, 312, 356, 389, 403, 438, 453, 490, 545, 610, 671, 716, 735, 782, 802, 851, 1007, 1085, 1142, 1166, 1250, 1311, 1503, 1598, 1667, 1696, 1767, 1870, 2098, 2177, 2210, 2291, 2325, 2408, 2528, 2792, 2883 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Numbers k such that k = (s+t)/2, where s is the square nearest to k, t is the triangular number nearest to k, and s != t. If there are two nearest triangular numbers, either of them is acceptable. - Edited by Robert Israel, Oct 07 2019 The sequence of roots of nearest squares begins: 1, 2, 5, 6, 7, 7, 8, 9, 12, 13, 13, 14, 14, 15, 16, 18, 19, 20, 20, 21, 21, ... The sequence of roots of nearest triangular numbers begins: 2, 3, 6, 7, 9, 10, 11, 13, 16, 17, 18, 19, 20, 21, 23, 24, 26, 27, 28, 29, ... The sequence of k-t (equals s-k) begins: -1, -1, 2, 4, 2, -3, -1, -5, 4, 8, -1, 3, -7, -3, -10, 12, 5, 11, -3, 3, -12, -6, ... LINKS Robert Israel, Table of n, a(n) for n = 1..10000 EXAMPLE 5 is in the sequence because 6 and 4 are the triangular number and square nearest to 5, and 5 = (6+4)/2. 23 is in the sequence because 21 and 25 are the triangular number and square nearest to 23, and 23 = (21+25)/2. MAPLE f:= proc(y) local t, x, s, r, R; t:= y*(y+1)/2; R:= NULL; for x from ceil(sqrt(t))-1 to floor(sqrt(t))+1 do s:= x^2; if s = t then next elif s < t then if t-y > s then next fi else if t+y+1 < s then next fi fi; r:= (s+t)/2; if r::integer then R:= R, r fi od; R end proc: map(f, [\$1..200]; # Robert Israel, Oct 06 2019 MATHEMATICA f[y_] := Module[{t, x, s, r, R = Nothing}, t = y(y+1)/2; For[x = Ceiling[Sqrt[t]]-1, x <= Floor[Sqrt[t]]+1, x++, s = x^2; Which[s == t, Continue[], s < t, If[t - y > s, Continue[]], True, If[t + y + 1 < s, Continue[]]]; r = (s + t)/2; If[IntegerQ[r], R = r] ]; R]; Map[f, Range[200]] (* Jean-François Alcover, Jul 30 2023, after Robert Israel *) PROG (Java) import java.math.*; public class A233074 { public static void main (String[] args) { for (long n = 1; ; n++) { // ok for small n long r2 = (long)Math.sqrt(n), b2 = r2*r2, a2 = (r2+1)*(r2+1); long t = (long)Math.sqrt(2*n), b3 = t*(t+1)/2, a3 = b3 + t + 1; if (b3 > n) { a3 = b3; b3 = t*(t-1)/2; } if ((b2+a3 == n*2 && n - b2 <= a2 - n && a3 - n <= n - b3) || (b3+a2 == n*2 && n - b3 <= a3 - n && a2 - n <= n - b2)) System.out.printf("%d, ", n); } } } CROSSREFS Cf. A000217, A000290, A233075. Sequence in context: A038919 A141181 A191030 * A100031 A293213 A215278 Adjacent sequences: A233071 A233072 A233073 * A233075 A233076 A233077 KEYWORD nonn,easy AUTHOR Alex Ratushnyak, Dec 03 2013 EXTENSIONS Corrected by Alex Ratushnyak, Jun 08 2014 STATUS approved

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Last modified July 15 23:38 EDT 2024. Contains 374343 sequences. (Running on oeis4.)