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A217286
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Larger of pairs of emirps (A006567) whose difference with the (smaller) reversal is a triangular number (A000217).
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3
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73, 1811, 7817, 7927, 11701, 12611, 14431, 14831, 15101, 15241, 15541, 15601, 16111, 16451, 16651, 17021, 18671, 18731, 19181, 19541, 19681, 19841, 32213, 32713, 33223, 33623, 33923, 35803, 36013, 36353, 36913, 37123, 37363, 37463, 37963, 39383, 39983, 71707
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OFFSET
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1,1
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COMMENTS
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Related to A217386 as triangular numbers (A000217) are to squares (A000290), but in the current sequence, only the larger of each emirp pair is used.
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LINKS
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FORMULA
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EXAMPLE
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a(1) = 73 because 73 - 37 = 36 = 8th triangular number.
a(2) = 1811 because R(1811) = A004086(1811) = 1181, and 1811 - 1181 = 630 = 35th triangular number.
a(3) = 7817 because 7817 - 7187 = 630 = 35th triangular number.
a(4) = 7927 because 7927 - 7297 = 630 = 35*36/2.
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MATHEMATICA
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emrp[p_]:=Module[{rev=IntegerReverse[p]}, If[rev!=p&&PrimeQ[rev], Max[ rev, p], Nothing]]; Select[Union[emrp/@Prime[Range[7500]]], OddQ[Sqrt[8(#-IntegerReverse[ #])+1]]&] (* Harvey P. Dale, Jan 30 2023 *)
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CROSSREFS
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KEYWORD
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nonn,base,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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