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A158864 Prepending 6 to n-th triangular number produces a prime. 1


%S 13,37,73,118,166,217,313,433,457,493,538,541,553,601,613,637,706,826,

%T 853,898,958,1042,1057,1138,1222,1357,1441,1513,1666,1681,1777,1858,

%U 1966,1993,2173,2281,2341,2422,2446,2518,2626,2638,2653,2686,2701,2737

%N Prepending 6 to n-th triangular number produces a prime.

%C Or, concatenated T(3) and T(n) are prime. Or, concatenated A000217(3) and A000217(n) are prime.

%H Harvey P. Dale, <a href="/A158864/b158864.txt">Table of n, a(n) for n = 1..1000</a>

%e n=13: T(13)=91, 691 is prime,

%e n=37: T(37)=703, 6703 is prime,

%e n=73: T(73)=2701, 62701 is prime.

%t Rest[Flatten[Position[Accumulate[Range[3000]],_?(PrimeQ[ 6*10^IntegerLength[ #]+#]&)]]] (* _Harvey P. Dale_, Jul 27 2015 *)

%Y A158844 Concatenated triangular numbers T(n), T(n + 1) and T(n + 2) are prime. A158862 Prepending 3 to n-th triangular number produces a prime. A158845 Prepending 1 to n-th triangular number produces a prime. A158750 Concatenated triangular numbers that are prime. A000217 Triangular numbers.

%K base,nonn

%O 1,1

%A _Zak Seidov_, Mar 28 2009

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Last modified January 27 17:58 EST 2020. Contains 331296 sequences. (Running on oeis4.)