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A063680
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Solutions to sigma(k) + 7 = sigma(k+7).
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2
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OFFSET
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1,1
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COMMENTS
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No other solutions < 4290000000. Sequence A063679 shows how to generate more solutions, but there may be solutions other than those produced by A063679.
Each of the first 4 terms of the sequence is of the form k = 9^j - 7:
74 = 9^2 - 7,
531434 = 9^6 - 7,
387420482 = 9^9 - 7,
2541865828322 = 9^13 - 7.
The next terms of this form are 9^53 - 7 and 9^82 - 7.
Does the sequence contain any terms that are not of this form?
(End)
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LINKS
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EXAMPLE
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sigma(74) + 7 = 121 = sigma(74+7), so 74 is in the sequence.
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PROG
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(PARI) isok(k) = sigma(k) + 7 == sigma(k+7); \\ Michel Marcus, Oct 25 2022
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CROSSREFS
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KEYWORD
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nonn,bref,hard,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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