seq=: sisyphuseq 1e6
   10 10$seq   NB. first 100 members of sequence
  1   3  6   3  8   4   2   1   8  4
  2   1 12   6  3  16   8   4   2  1
 18   9 28  14  7  30  15  44  22 11
 42  21 58  29 70  35  78  39  86 43
 96  48 24  12  6   3  62  31  92 46
 23  90 45 116 58  29 102  51 130 65
148  74 37 126 63 160  80  40  20 10
  5 106 53 156 78  39 146  73 182 91
204 102 51 178 89 220 110  55 192 96
 48  24 12   6  3 142  71 220 110 55
   x:(,. (seq {~ <:),. P p:@{~ <:) 1e3 1e4 1e5 1e6  NB. nth elements of sequence and corresponding largest prime used
   1000     990    2273
  10000   24975   30713
 100000  265781  392111
1000000 8820834 4761697