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A217286
Larger of pairs of emirps (A006567) whose difference with the (smaller) reversal is a triangular number (A000217).
3
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
OFFSET
1,1
COMMENTS
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.
LINKS
FORMULA
{k: k is in A006567 and k - A004086(k) is in A000217}.
EXAMPLE
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.
MATHEMATICA
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 *)
CROSSREFS
KEYWORD
nonn,base,easy
AUTHOR
Jonathan Vos Post, Oct 02 2012
EXTENSIONS
More terms from Alois P. Heinz, Oct 03 2012
STATUS
approved