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 A175424 a(n) = number of steps of iterations of {(((D_k^D_(k-1))^D_(k-2))^...)^D_1, where D_k = k-th digit D of number r and k = the number digit of number r in decimal expansion of r (A055642)} needed to reach a single-digit number starting at r = n, or a(n) = -1 if a single-digit number is never reached. 7
 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, -1, -1, 3, 3, 2, 1, 1, 1, 4, 3, -1, -1, 3, 3, 3, 1, 1, 2, 2, 3, 3, 3, 2, 2, 2, 1, 1, 2, 3, 2, 4, 3, 2, 3, 2, 1, 1, 3, 3, 2, 3, 3, 3, 3, 2, 1, 1, 4, 3, 3, 3, 3, 3, 2, 3, 1, 1, 3, 2, 3, 2, 3, 2, 2, 2, 1, 1, 3, 3, 3, 3, 2, 2, 2, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,25 COMMENTS Conjecture: max(a(n)) = 4. LINKS EXAMPLE For n = 33: a(33) = 4 because for the number 33 there are 4 steps of defined iteration: {3^3 = 27}, {7^2 = 49}, {9^4 = 6561}, {((1^6)^5)^6 = 1}. CROSSREFS Cf. A175419, A175420, A175421, A175422, A175423, A175425, A175426, A175427. Sequence in context: A100640 A242779 A215065 * A144401 A034929 A099509 Adjacent sequences:  A175421 A175422 A175423 * A175425 A175426 A175427 KEYWORD sign,base AUTHOR Jaroslav Krizek, May 09 2010 STATUS approved

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Last modified June 17 17:01 EDT 2021. Contains 345085 sequences. (Running on oeis4.)