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 A110701 Friendly run sums: numbers S with two run sums (sum of positive integer runs) that share one common number, i.e., S = a + (a+1) + ... + b = b + (b+1) + ... + c with a < b < c. 2
 9, 21, 30, 42, 65, 70, 99, 105, 117, 133, 135, 154, 175, 180, 225, 231, 275, 285, 341, 342, 345, 364, 385, 414, 440, 450, 455, 481, 495, 540, 546, 567, 630, 645, 675, 693, 744, 750, 765, 825, 833, 936, 945, 990, 1035, 1045, 1140, 1161, 1170, 1176, 1178 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The sums are the difference of two triangular numbers A000217. The common numbers themselves are A094550. The sums, n > 0, are n = (b-a+1)(a+b)/2 = (b+c)(c-b+1)/2 where b^2 + a - a^2 = c^2 + c - b^2, is solvable in integers for 0 < a < b < c. Since the runs have something in common, they are "friends". The series of sums *without* the common number is A110702. The numbers common to the two runs are A094550. LINKS Ron Knott Runsums T. Verhoeff, Rectangular and Trapezoidal Arrangements, J. Integer Sequences, Vol. 2, 1999, #99.1.6. EXAMPLE 2 + 3 + 4 = 4 + 5 share the common number 4. The sum of both is 9 and this is the smallest such sum with a common "friend" (4), so a(1)=9. CROSSREFS Cf. A001227, A094550, A110702, A110703. Sequence in context: A045252 A139538 A173460 * A243703 A133929 A325573 Adjacent sequences:  A110698 A110699 A110700 * A110702 A110703 A110704 KEYWORD nonn AUTHOR Ron Knott, Aug 04 2005 STATUS approved

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Last modified July 6 11:37 EDT 2022. Contains 355110 sequences. (Running on oeis4.)