cp's OEIS Frontend

A221491 Number of primes of the form k*n - k + n, 1 <= k <= n.

%I A221491 #13 Jan 26 2022 12:00:16
%S A221491 0,1,2,2,2,2,5,2,3,3,5,3,6,4,7,6,5,4,9,2,8,7,8,4,11,5,10,8,10,4,16,3,
%T A221491 10,10,12,9,15,4,10,11,13,6,20,7,11,10,15,8,19,6,19,13,16,5,25,10,19,
%U A221491 10,15,9,26,7,16,13,16,13,30,9,18,14,26,10,25,10
%N A221491 Number of primes of the form k*n - k + n, 1 <= k <= n.
%C A221491 a(n) = sum(A010051(A162610(n,k)): k=1..n) = number of primes in n-th row of the triangle in A162610. - _Reinhard Zumkeller_, Jan 19 2013
%H A221491 Reinhard Zumkeller, <a href="/A221491/b221491.txt">Table of n, a(n) for n = 1..1000</a>
%e A221491 Row 10 of A162610 = [19,28,37,46,55,64,73,82,91,100] containing three primes: [19,37,73], therefore a(10) = 3;
%e A221491 row 11 of A162610 = [21,31,41,51,61,71,81,91,101,111,121] containing five primes: [31,41,61,71,101], therefore a(11) = 5.
%t A221491 a[n_] := Sum[Boole[PrimeQ[(k+1)(n-1)+1]], {k, 1, n}];
%t A221491 Table[a[n], {n, 1, 100}] (* _Jean-François Alcover_, Sep 20 2021 *)
%o A221491 (Haskell)
%o A221491 a221491 n = sum [a010051' (k*n - k + n) | k <- [1..n]]
%o A221491 (PARI) a(n) = sum(k=1, n, isprime(k*n - k + n)); \\ _Michel Marcus_, Jan 26 2022
%Y A221491 Cf. A221490.
%Y A221491 Cf. A010051, A162610.
%K A221491 nonn,tabl
%O A221491 1,3
%A A221491 _Reinhard Zumkeller_, Jan 19 2013