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 A072732 Simple triangle-stretching N X N -> N bijection: push terms in the middle twice as far down to make space for the terms obtained by bisecting the edges, which are thus contracted by the same factor. Do this for all "triangle-shells" successively contained inside each other in A001477. 7
 0, 1, 4, 7, 12, 2, 3, 17, 24, 8, 11, 31, 40, 18, 5, 6, 23, 49, 60, 32, 13, 16, 39, 71, 84, 50, 25, 9, 10, 30, 59, 97, 112, 72, 41, 19, 22, 48, 83, 127, 144, 98, 61, 33, 14, 15, 38, 70, 111, 161, 180, 128, 85, 51, 26, 29, 58, 96, 143, 199, 220, 162, 113, 73, 42, 20, 21, 47 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS PROG (Scheme) (define (A072732 n) (packA072732 (A025581 n) (A002262 n))) (define (packA001477 x y) (/ (+ (expt (+ x y) 2) x (* 3 y)) 2)) (define (packA072732 x y) (let ((x-y (- x y))) (cond ((<= x-y 0) (packA001477 (+ (* 2 x) (modulo x-y 2)) (+ (* 2 x) (floor->exact (/ (1+ (- x-y)) 2))))) (else (packA001477 (+ (* 2 (1+ y)) (floor->exact (/ (- x-y 2) 2))) (+ (* 2 y) (modulo (1+ x-y) 2))))))) CROSSREFS Inverse: A072733, projections: A072736 & A072737, variant of the same theme: A072734. Cf. also A001477 and its projections A025581 & A002262. Sequence in context: A128836 A061956 A178939 * A270684 A083487 A253129 Adjacent sequences:  A072729 A072730 A072731 * A072733 A072734 A072735 KEYWORD nonn,tabl AUTHOR Antti Karttunen, Jun 12 2002 STATUS approved

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Last modified August 15 18:27 EDT 2018. Contains 313779 sequences. (Running on oeis4.)