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URL: https://oeis.org/A158844

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A158844
Numbers k such that the concatenation of triangular numbers T(k), T(k+1) and T(k+2) is prime.
3
20, 31, 35, 39, 44, 99, 135, 139, 155, 164, 200, 211, 271, 275, 284, 340, 359, 360, 404, 416, 424, 444, 484, 496, 511, 564, 596, 611, 640, 676, 724, 800, 836, 859, 860, 871, 876, 884, 919, 940, 944, 951, 971, 976, 995, 1000, 1004, 1064, 1116, 1131, 1144, 1159
OFFSET
1,1
EXAMPLE
T(20) = 210, T(21) = 231, T(22) = 253, and 210231253 is prime, so 20 is a term;
T(31) = 496, T(32) = 528, T(33) = 561, and 496528561 is prime, so 31 is a term.
MATHEMATICA
p3cQ[n_]:=Module[{c1=(n(n+1))/2, c2=((n+1)(n+2))/2, c3=((n+2)(n+3))/2}, PrimeQ[FromDigits[Flatten[IntegerDigits/@{c1, c2, c3}]]]]; Select[Range[ 1250], p3cQ] (* Harvey P. Dale, Sep 13 2011 *)
CROSSREFS
Sequence in context: A167360 A113760 A053679 * A077340 A077343 A218793
KEYWORD
nonn,base
AUTHOR
Zak Seidov, Mar 28 2009
STATUS
approved