1 0 2 1 3 3 4 4 5 5 6 7 7 8 8 13 9 16 10 17 11 19 12 31 13 61 14 89 15 107 16 127 17 521 18 607 19 1279 20 2203 21 2281 22 3217 23 4253 24 4423 25 9689 26 9941 27 11213 28 19937 29 21701 30 23209 31 44497 32 86243 33 110503 34 132049 35 216091 36 756839 37 859433 38 1257787 39 1398269 40 2976221 41 3021377 42 6972593 43 13466917 44 20996011 45 24036583 46 25964951 47 30402457 48 32582657 49 37156667 50 42643801 51 43112609 52 57885161 # Numbers 74207281, 77232917, and 82589933 also occur, but it is not fully proved that an undiscovered odd member # cannot occur as a(53), a(54), or a(55). See A000043.