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 A108581 Positive triangular numbers repeated their own number of times. 0
 1, 3, 3, 3, 6, 6, 6, 6, 6, 6, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS In terms of the repetition convolution operator #, where (sequence A) # (sequence B) = the sequence consisting of A(n) copies of B(n), then this sequence is the repetition self-convolution (A000217(n)-0) # (A000217(n)-0). Over the set of positive infinite integer sequences, # gives a nonassociative noncommutative groupoid with a left identity (A000012) but no right identity, where the left identity is also a right nullifier and idempotent. LINKS FORMULA a(1) = 1, for n>1: a(A000217(n)-1) = a(A000217(n)) = ... = a(A000217(n+1)-2) = A000217(n). CROSSREFS Cf. A000217, A002024, A072649, A074279. Sequence in context: A112669 A098529 A133774 * A073080 A171601 A057944 Adjacent sequences:  A108578 A108579 A108580 * A108582 A108583 A108584 KEYWORD easy,nonn AUTHOR Jonathan Vos Post, Jul 25 2005 STATUS approved

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Last modified June 24 18:43 EDT 2021. Contains 345419 sequences. (Running on oeis4.)