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%I #8 Mar 04 2018 03:05:55
%S 1,3,21,91,153,171,231,253,561,741,1081,1653,1711,1891,2211,2701,3321,
%T 3403,4851,6441,8001,9453,9591,10153,12403,15051,16471,17391,18721,
%U 23653,24753,27261,30381,32131,35511,38781,39621,41041,44253,45753,51681,55611,56953
%N Triangular numbers t such that t+10 is a prime.
%C The only triangular numbers t such that t-10 is a prime are 15 and 21.
%p t1:=[]; f:=n->n*(n+1)/2; for n from 1 to 1000 do if isprime(f(n)+10) then t1:=[op(t1),f(n)]; fi; od: t1; # _N. J. A. Sloane_, Jun 01 2007
%t Select[Table[n(n+1)/2,{n,400}],PrimeQ[ #+10]&]
%o (PARI) for(k=1, 100, t=(k*(k+1))/2; if(isprime(t+10), print1(t, ", "))) \\ _Joerg Arndt_, Jun 01 2007
%K nonn
%O 1,2
%A _Zak Seidov_, May 15 2007
%E Checked by _N. J. A. Sloane_, Jun 01 2007
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