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| n | a(n) |
|---|---|
| 1 | 47 |
| 2 | 47447747 |
| 3 | 77474447 |
| 4 | 4447747747 |
| 5 | 4744477747 |
| 6 | 4744747477 |
| 7 | 4744747747 |
| 8 | 4747444777 |
| 9 | 4747747447 |
| 10 | 4774744477 |
| 11 | 4774747447 |
| 12 | 7444477477 |
| 13 | 7447474477 |
| 14 | 7744744747 |
| 15 | 7744747447 |
| 16 | 7747444477 |
| 17 | 7747447447 |
| 18 | 7747744447 |
| 19 | 44444477777477 |
| 20 | 44447447774777 |
| 21 | 44447474777477 |
[47,47447747,77474447,4447747747,4744477747, 4744747477,4744747747,4747444777,4747747447, 4774744477,4774747447,7444477477,7447474477, 7744744747,7744747447,7747444477,7747447447, 7747744447,44444477777477,44447447774777, 44447474777477] |