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A094534
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Centered hexamorphic numbers: the k-th centered hexagonal number, 3k(k-1)+1, ends in k.
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1
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1, 7, 17, 51, 67, 167, 251, 417, 501, 667, 751, 917, 1251, 1667, 5001, 5417, 6251, 6667, 10417, 16667, 50001, 56251, 60417, 66667, 166667, 260417, 406251, 500001, 666667, 760417, 906251, 1406251, 1666667, 5000001, 5260417, 6406251, 6666667, 16666667
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OFFSET
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1,2
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COMMENTS
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Given any number in the sequence, if you remove one or more digits from the beginning you always get another number in the sequence. This makes it easy to find higher terms -- just take an existing term and try adding a digit (with perhaps additional 0's) at the beginning. For example, to 6251 prepend 5 to get a 5-digit term, or 40 or 90 to get a 6-digit term.
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LINKS
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FORMULA
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10^(d-1) <= n < 10^d; 3n(n-1)+1 == n mod 10^d
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EXAMPLE
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417 is in the sequence because if n=417, 3n(n-1)+1=520417, which ends in 417.
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PROG
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(PARI) isok(n) = {my(m = 3*n*(n-1)+1); (m - n) % 10^#Str(n) == 0; } \\ Michel Marcus, Jun 21 2018
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CROSSREFS
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KEYWORD
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base,easy,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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