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EXAMPLE
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a(k) = 0 for k<7, since no shorter string can be obtained by duplication of substrings.
a(7) = 1 = #{abcdefg},
a(8) = 7 = #{aabcdefg, abbcdefg, abccdefg, abcddefg, abcdeefg, abcdeffg, abcdefgg},
a(9) = 8*(8+1)/2-2 = 34:
for each letter we have one string of the form aaabcdefg;
for each 2-element subset {a,b}, {a,c}, ... we have the string with each of these two letters duplicated (i.e., aabbcdefg, aabccdefg, ...);
and for each of ab,bc,cd,...,fg we have the string with this substring duplicated (ababcdefg,...,abcdefgfg).
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