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This is my sixth batch of sequence puzzles that are nasty and hard to solve; yet, each of them has a clear and justifiable solution.

Sequence 1:
Ca, Cl, ??, Er, ??, Po, Te, Th, ??

Sequence 2:
A, G, ?, G, ?, W, V, E, G, E, ?, ?

Sequence 3:
..., 1906, 1912, 1918, 1936, 1938, ????, ????, 1978, ????, 1992, 1998, 2002, 2004, 2008, ...

Sequence 4:
?, ?, ?, R, A, O, T, E, L, A, I, D, L, N, A, S, Y, A, E, D, ...

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  • $\begingroup$ is each of them a puzzle or are they combined? $\endgroup$
    – user10001
    Commented Mar 15, 2015 at 19:52
  • $\begingroup$ Sequence 1 consist entirely from elements from the periodic table of elements. The numbers of the elements are 20, 17, ??, 68, ??, 84, 52, 90, ?? . However, I still cannot find the connection between those numbers. Am I on the right track? $\endgroup$
    – zlobi.wan.kenobi
    Commented Mar 15, 2015 at 21:27
  • $\begingroup$ Shouldn't this be posted as 4 separate puzzles? What if someone correctly answers Sequences 1 and 2, and another correctly answers Sequences 3 and 4. Who gets the credit then? Is it possible to place the green check mark on two answers? $\endgroup$
    – qzx
    Commented Mar 16, 2015 at 3:15
  • 1
    $\begingroup$ Once again the prize goes to Len. $\endgroup$
    – Alexis
    Commented Mar 18, 2015 at 16:10

2 Answers 2

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Sequence 1 is a list of the:

nine muses of Greek mythology as follows (bold for missing letters):

Calliope, Clio, Euterpe, Erato, Melpomene, Polyhymnia, Terpsichore, Thalia, Urania

Sequence 2 is the order of the:

British monarchs as follows (bold for missing letters):

Anne, George I, George II, George III, George IV, William IV, Victoria, Edward VII, George V, Edward VIII, George VI, Elizabeth II

Sequence 4 is a list of the:

second letters of the abbreviation of US states (bold for missing letters).
The order is alphabetical based on the full state name.

AL, AK, AZ, AR, CA, CO, CT, DE, FL, GA, HI, ID, IL, IN, IA, KS, KY, LA, ME, MD...

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Sequence 3:

looks like years, but actually is primes plus five.

The page http://primes.utm.edu/lists/small/10000.txt lists the first 10.000 primes:
..., 1901, 1907, 1913, 1931, 1933, 1949, 1951, 1973, 1979, 1987, 1993, 1997, 1999, 2003, ...
By adding 5 to each number, this list turns into
..., 1906, 1912, 1918, 1936, 1938, 1954, 1956, 1978, 1984, 1992, 1998, 2002, 2004, 2008, ...

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