40 Years with Sloane’s Integer Sequences Jeffrey Shallit School of Computer Science, University of Waterloo Waterloo, Ontario N2L 3G1, Canada
[email protected] http://www.cs.uwaterloo.ca/~shallit 1/1 My Christmas Present in 1974 2/1 Sloane’s First Letter to Me 3/1 Modified Engel expansions Kalpazidou, Knopfmacher, and Knopfmacher (1990) studied expansions of the form 1 1 1 1 1 1 α = a0 + + a1 − a1 + 1 · a2 a1 + 1 · a2 + 1 · a3 −··· for real numbers α, where the ai are non-negative integers with ai positive for i 1. ≥ This expansion is essentially unique provided ai ai . +1 ≥ I tried expanding α = 2/5 and found doubly-exponential growth (a0, a1,...)=(0, 2, 3, 7, 13, 97, 193, 18817, 37633, 708158977,...). Although this particular sequence was not in the OEIS at the time, its even-indexed subsequence (2, 7, 97, 18817, 708158977,...) certainly was (as A002812) and led to the discovery that ... 4/1 Modified Engel expansions 1 2i 2i a i = (2 + √3) + (2 √3) 2 +1 2 − and 2i 2i a i = (2+ √3) + (2 √3) 1. 2 +2 − − From this it was easy to deduce the general form of the expansion for 2/(2r + 1). Compare Aho and Sloane’s 1973 paper, “Some Doubly Exponential Sequences”. Open Problem: find a simple closed form for the expansion of 3/7. 5/1 Some confusing quotients Suppose we set c = a, c = b, and for n 2 define cn to be the 0 1 ≥ smallest integer such that c c n > n−1 .