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 A158845 Prepending 1 to n-th triangular number produces a prime. 3
 1, 2, 13, 17, 18, 21, 38, 41, 62, 66, 77, 97, 98, 106, 117, 118, 133, 146, 153, 157, 161, 178, 181, 197, 198, 202, 206, 217, 222, 226, 233, 237, 242, 257, 261, 266, 286, 297, 301, 302, 318, 321, 322, 338, 346, 362, 373, 377, 393, 402, 413, 421, 422, 453, 461, 462 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Or, concatenated T(1) and T(n) produces a prime, or, concatenated A000217(1) and A000217(n) produces a prime. LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 EXAMPLE n=2: T(2)=3, 13 is prime, n=13: T(13)=91, 191 is prime, n=17: T(17)=153, 1153 is prime. MATHEMATICA Rest[Select[Range[600], PrimeQ[FromDigits[Join[{1}, IntegerDigits[(# (#+1))/2]]]]&]] [From Harvey P. Dale, Apr 15 2009] trprQ[n_]:=Module[{tr=(n(n+1))/2}, PrimeQ[10^IntegerLength[tr]+tr]]; Select[ Range[ 500], trprQ] (* Harvey P. Dale, Aug 04 2019 *) CROSSREFS A158844 Concatenated triangular numbers T (n), T(n + 1) and T(n + 2) are prime. A158750 Concatenated triangular numbers that are prime. A000217 Triangular numbers. Sequence in context: A258317 A318911 A065245 * A163786 A153507 A124277 Adjacent sequences:  A158842 A158843 A158844 * A158846 A158847 A158848 KEYWORD nonn,base AUTHOR Zak Seidov, Mar 28 2009 EXTENSIONS First term (1) prepended by Harvey P. Dale, Aug 04 2019 STATUS approved

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Last modified January 22 19:47 EST 2020. Contains 331153 sequences. (Running on oeis4.)