**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.**

*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*

*1 seqfan posts*

Index of A-numbers in seqfan: by ascending order by month by frequency by keyword

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