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Date: 20-7-2020
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Date: 17-8-2020
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Date: 11-8-2020
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A number which is simultaneously a nonagonal number and pentagonal number
and therefore satisfies the Diophantine equation
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(1) |
Completing the square and rearranging gives
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(2) |
Defining and
gives the Pell-like equation
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(3) |
This has solutions in corresponding to solutions which are integral in
and
of
, (14, 21), (7189, 10981), (165026, 252081), (86968201, 132846121), ... (OEIS A048913 and A048914), giving the nonagonal pentagonal numbers 1, 651, 180868051, 95317119801, 26472137730696901, ... (OEIS A048915).
REFERENCES:
Sloane, N. J. A. Sequences A048913, A048914, and A048915 in "The On-Line Encyclopedia of Integer Sequences."
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