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 A283681 Unique sequence with a(1)=1, a(2)=2, representing an array read by antidiagonals in which the i-th row is this sequence itself multiplied by i. 5
 1, 2, 2, 2, 4, 3, 2, 4, 6, 4, 4, 4, 6, 8, 5, 3, 8, 6, 8, 10, 6, 2, 6, 12, 8, 10, 12, 7, 4, 4, 9, 16, 10, 12, 14, 8, 6, 8, 6, 12, 20, 12, 14, 16, 9, 4, 12, 12, 8, 15, 24, 14, 16, 18, 10, 4, 8, 18, 16, 10, 18, 28, 16, 18, 20, 11, 4, 8, 12, 24, 20, 12, 21, 32, 18, 20, 22, 12, 6, 8 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Any integer greater than 1 appears infinitely many times. In particular, any n appears at the position (n^2 + n)/2. For prime n > 2, this is its first appearance; for composite n, it is not the first. 2 appears at the positions 2, 3, 4, 7, 22, 232, 26797, ... (A007501(n) + 1). When the sequence is considered as an array, any prime n appears only in the first row (infinitely many times) and in the first column (once). LINKS Ivan Neretin, Table of n, a(n) for n = 1..26796 FORMULA a((n^2+n)/2)=n. EXAMPLE The sequence begins: 1, 2, 2, 2, 4, 3, 2, 4, 6, 4, ... It represents a rectangular array read by downward antidiagonals. The first row of the array is this very sequence itself. The second row is this sequence multiplied by 2, and so on:   1  2  2  2  4  3 ...   2  4  4  4  8  ...   3  6  6  6  ...   4  8  8  ...   5 10  ...   6 ...   ... MATHEMATICA Nest[Flatten@Table[#[[n - i]]*i, {n, Length[#] + 1}, {i, n - 1}] &, {1, 2}, 4] CROSSREFS Cf. A007501 (number of terms produced by the Mathematica code after n iterations). Cf. A283682, A283683. Sequence in context: A054709 A121806 A056944 * A222819 A194319 A208609 Adjacent sequences:  A283678 A283679 A283680 * A283682 A283683 A283684 KEYWORD nonn,tabl,nice,look AUTHOR Ivan Neretin, Mar 14 2017 STATUS approved

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Last modified May 12 11:55 EDT 2021. Contains 343821 sequences. (Running on oeis4.)