ok .. it is the first 231 actually ... computed in algol68g as : PR heap=256M PR MODE SERIES = FLEX [ 1 : 0] INT; # Initially, no elements # OP LAST = ( SERIES r) INT: r[ UPB r]; # Last element of a series # PROC rob number = ( INT n) INT: # The n-th Rob number # CASE n IN 1, 3, 4, 5 # First 4 in a table # OUT SERIES r := 1, # Series, initially one element # INT m3 := 3, m4 := 4, m5 := 5, # Multipliers # INT i := 1, j := 1, k := 1; # Counters # TO n - 1 DO OP MIN = ( INT i, j) INT: ( i < j | i | j); PRIO MIN = 9; OP +:= = ( REF SERIES s, INT elem) VOID: # Extend a series by one element, only keep the elements you need # ( INT lwb = i MIN j MIN k, upb = UPB s; REF SERIES new s = NEW FLEX [ lwb : upb + 1] INT; ( new s[ lwb : upb] := s[ lwb : upb], new s[ upb + 1] := elem ); s := new s ); # Determine the n-th rob number iteratively # r +:= m3 MIN m4 MIN m5; ( LAST r = m3 | m3 := 3 * r[ i +:= 1]); ( LAST r = m4 | m4 := 4 * r[ j +:= 1]); ( LAST r = m5 | m5 := 5 * r[ k +:= 1]) OD; LAST r ESAC; FOR i TO 231 DO print( ( whole( i , -3), " ", whole( rob number( i), -7), new line ) ) OD # ---------------------------------- # 1 1 2 3 3 4 4 5 5 9 6 12 7 15 8 16 9 20 10 25 11 27 12 36 13 45 14 48 15 60 16 64 17 75 18 80 19 81 20 100 21 108 22 125 23 135 24 144 25 180 26 192 27 225 28 240 29 243 30 256 31 300 32 320 33 324 34 375 35 400 36 405 37 432 38 500 39 540 40 576 41 625 42 675 43 720 44 729 45 768 46 900 47 960 48 972 49 1024 50 1125 51 1200 52 1215 53 1280 54 1296 55 1500 56 1600 57 1620 58 1728 59 1875 60 2000 61 2025 62 2160 63 2187 64 2304 65 2500 66 2700 67 2880 68 2916 69 3072 70 3125 71 3375 72 3600 73 3645 74 3840 75 3888 76 4096 77 4500 78 4800 79 4860 80 5120 81 5184 82 5625 83 6000 84 6075 85 6400 86 6480 87 6561 88 6912 89 7500 90 8000 91 8100 92 8640 93 8748 94 9216 95 9375 96 10000 97 10125 98 10800 99 10935 100 11520 101 11664 102 12288 103 12500 104 13500 105 14400 106 14580 107 15360 108 15552 109 15625 110 16384 111 16875 112 18000 113 18225 114 19200 115 19440 116 19683 117 20480 118 20736 119 22500 120 24000 121 24300 122 25600 123 25920 124 26244 125 27648 126 28125 127 30000 128 30375 129 32000 130 32400 131 32805 132 34560 133 34992 134 36864 135 37500 136 40000 137 40500 138 43200 139 43740 140 46080 141 46656 142 46875 143 49152 144 50000 145 50625 146 54000 147 54675 148 57600 149 58320 150 59049 151 61440 152 62208 153 62500 154 65536 155 67500 156 72000 157 72900 158 76800 159 77760 160 78125 161 78732 162 81920 163 82944 164 84375 165 90000 166 91125 167 96000 168 97200 169 98415 170 102400 171 103680 172 104976 173 110592 174 112500 175 120000 176 121500 177 128000 178 129600 179 131220 180 138240 181 139968 182 140625 183 147456 184 150000 185 151875 186 160000 187 162000 188 164025 189 172800 190 174960 191 177147 192 184320 193 186624 194 187500 195 196608 196 200000 197 202500 198 216000 199 218700 200 230400 201 233280 202 234375 203 236196 204 245760 205 248832 206 250000 207 253125 208 262144 209 270000 210 273375 211 288000 212 291600 213 295245 214 307200 215 311040 216 312500 217 314928 218 327680 219 331776 220 337500 221 360000 222 364500 223 384000 224 388800 225 390625 226 393660 227 409600 228 414720 229 419904 230 421875 231 442368