Sequences

There are infinite formulas that will fit any finite series. Try to guess the following number in each sequence (using the most simple mathematical operations, because as I mentioned, there is more than one solution for each number sequence.

1. 8723, 3872, 2387, ?
2. 1, 4, 9, 18, 35, ?
3. 23, 45, 89, 177, ?
4. 7, 5, 8, 4, 9, 3, ?
5. 11, 19, 14, 22, 17, 25, ?
6. 3, 8, 15, 24, 35, ?
7. 2, 4, 5, 10, 12, 24, 27, ?
8. 1, 3, 4, 7, 11, 18, ?
9. 99, 92, 86, 81, 77, ?
10. 0, 4, 2, 6, 4, 8, ?
11. 1, 2, 2, 4, 8, 11, 33, ?
12. 1, 2, 6, 24, 120, ?
13. 1, 2, 3, 6, 11, 20, 37, ?
14. 5, 7, 12, 19, 31, 50, ?
15. 27, 82, 41, 124, 62, 31, 94, 47, 142, 71, 214, 107, ?
16. 126, 63, 190, 95, 286, 143, 430, 215, 646, 323, 970, ?
17. 4, 7, 15, 29, 59, 117, ?
18. 2, 3, 2, 3, 2, 4, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 4, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 4, 2, 3, 2, 3, 2, 5, ?
19. 4, 4, 341, 6, 4, 4, 6, 6, 4, 4, 6, 10, 4, 4, 14, 6, 4, 4, 6, 6, 4, 4, 6, 22, 4, 4, 9, 6, ?

Ans:

1. 8723, 3872, 2387, ? 7238 (moving of numerals)
2. 1, 4, 9, 18, 35, ? 68 (x*2+2, +1, +0, -1, -2)
3. 23, 45, 89, 177, ? 353 (x*2-1)
4. 7, 5, 8, 4, 9, 3, ? 10, 2 (two series - every second number: 7, 8, 9, 10 and 5, 4, 3, 2)
5. 11, 19, 14, 22, 17, 25, ? 20, 28 (two series - every second number: 11, 14, 17, 20 and 19, 22, 25, 28)
6. 3, 8, 15, 24, 35, ? 48 (x+5, +7, +9, +11, +13)
7. 2, 4, 5, 10, 12, 24, 27, ? 54, 58 (x*2, +1, *2, +2, *2, +3, *2, +4)
8. 1, 3, 4, 7, 11, 18, ? 29 (a+b=c, b+c=d, c+d=e)
9. 99, 92, 86, 81, 77, ? 74 (x-7, -6, -5, -4, -3)
10. 0, 4, 2, 6, 4, 8, ? 6 (x+4, -2, +4, -2, +4, -2)
11. 1, 2, 2, 4, 8, 11, 33, ?
12. 1, 2, 6, 24, 120, ? 720 (x*2, *3, *4, *5, *6)
13. 1, 2, 3, 6, 11, 20, 37, ?
14. 5, 7, 12, 19, 31, 50, ? 81 (a+b=c, b+c=d, c+d=e)
15. 27, 82, 41, 124, 62, 31, 94, 47, 142, 71, 214, 107, ? 322, 161 (x*3+1, /2, *3+1, /2)
16. 126, 63, 190, 95, 286, 143, 430, 215, 646, 323, 970, ? 485, 1456 (x/2, *3+1, /2, *3+1)
17. 4, 7, 15, 29, 59, 117, ? 235 (x*2-1, *2+1, *2-1)
18. 2, 3, 2, 3, 2, 4, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 4, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 4, 2, 3, 2, 3, 2, 5, ? 2, 3, 2, 3, 2, 4, 2, 3, 2, 3, 2, 5
19. 4, 4, 341, 6, 4, 4, 6, 6, 4, 4, 6, 10, 4, 4, 14, 6, 4, 4, 6, 6, 4, 4, 6, 22, 4, 4, 9, 6, ? 4, 4