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COMMENTS
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If n = p^k, with p an odd prime, then a(n) = 1 + 2^n + 3^n. We also have
a(12) <= 2387003305930334914 (with divisors 1, 2, 17, 34),
a(14) = 100006103532010 (1, 2, 5, 10),
a(15) = 14381676 (1, 2, 3),
a(16) <= 1880100018939820249188604888836, (1, 2, 3, 4, 6, 12, 13, 26, 39, 52, 78),
a(18) = 1000003814697527770 (1, 2, 5, 10),
a(20) <= 19105043663614041367780, (1, 2, 4, 5, 7, 10, 13),
a(21) = 10462450356 (1, 2, 3),
a(22) = 10000002384185795209930, (1, 2, 5, 10), and
a(24) <= 226500219158007133816826003223992308820431641700, (1, 2, 4, 5, 10, 20, 25, 47, 50, 94).
In general, if n = 4*k+2, then a(n) <= 1 + 2^n + 5^n + 10^n. (End)
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