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A131317
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Repdigit semiprimes.
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1
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22, 33, 55, 77, 111, 1111, 11111, 1111111, 11111111111, 11111111111111111, 2222222222222222222, 3333333333333333333, 5555555555555555555, 7777777777777777777, 22222222222222222222222, 33333333333333333333333
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OFFSET
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1,1
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COMMENTS
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A semiprime can be repdigit (base 10) in only two ways. It can be a repunit semiprime (A102782) or it can be a repunit prime times a prime digit {2, 3, 5, 7}. Subset of A046328. Subset of A116063. Occurs in proof that the sequence is infinite in which a(n) is the least semiprime > a(n-1) such that a(n) has no digit in common with a(n-1).
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LINKS
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Table of n, a(n) for n=1..16.
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FORMULA
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A102782 UNION 2*A004022 UNION 3*A004022 UNION 5*A004022 UNION 7*A004022.
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EXAMPLE
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a(1) = 22 = 2 * 11;
a(2) = 33 = 3 * 11;
a(3) = 55 = 5 * 11;
a(4) = 77 = 7 * 11;
a(5) = 111 = 3 * 37;
a(6) = 1111 = 11 * 101;
a(7) = 11111 = 41 * 271.
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CROSSREFS
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Cf. A000042, A001358, A004023, A046413, A046328, A102782, A116063.
Sequence in context: A120146 A138840 A116063 * A176597 A067087 A155709
Adjacent sequences: A131314 A131315 A131316 * A131318 A131319 A131320
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KEYWORD
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base,easy,nonn
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AUTHOR
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Jonathan Vos Post, Oct 21 2007
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STATUS
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approved
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