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A253827 a(n) is the number of primes of the form x^2 + x + prime(n) for 0 <= x <=prime(n). 2
1, 2, 4, 4, 10, 4, 16, 6, 10, 13, 14, 16, 40, 8, 26, 19, 34, 21, 36, 28, 18, 18, 34, 27, 31, 68, 16, 71, 30, 23, 37, 37, 67, 44, 54, 55, 54, 26, 65, 50, 70, 68, 79, 43, 60, 70, 52, 51, 132, 38, 60, 100, 59, 111, 114, 84, 77, 68, 78, 105, 49, 67, 124, 145, 35 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Equivalently, number of distinct primes of the form x^2 - x + prime(n) for 0 <= x <= prime(n). (The point is that x^2 + x = (x+1)^2 - (x+1), so the two forms give the same numbers. x^2 - x + prime(n) is the same for x=0 and x=1, which is why the "distinct" in the comment. - Robert Israel, Oct 09 2016)

1 <= a(n) <= prime(n)-1.  a(n) = prime(n)-1 iff n is in A014556.  Are there any n > 1 such that a(n) = 1? - Robert Israel, Jan 16 2015

LINKS

Michel Lagneau, Table of n, a(n) for n = 1..4000

Eric Weisstein's World of Mathematics, Lucky Number of Euler

EXAMPLE

a(13) = 40 because prime(13) = 41 and x^2 + x + 41 generates 40 prime numbers for x = 0..41.

MAPLE

f:= proc(n)

local p, x;

p:= ithprime(n);

nops(select(isprime, [seq(x^2+x+p, x=0..p)]))

end proc:

seq(f(n), n=1..100); # Robert Israel, Jan 16 2015

MATHEMATICA

lst={}; Do[p=Prime[n]; k=0; Do[If[PrimeQ[x^2+x+p], k=k+1], {x, 0, p}]; AppendTo[lst, k], {n, 1, 100}]; lst

Table[With[{p=Prime[n]}, Count[Table[x^2+x+p, {x, 0, p}], _?PrimeQ]], {n, 70}] (* Harvey P. Dale, May 27 2018 *)

PROG

(PARI) a(n) = my(p=prime(n)); sum(k=0, p, isprime(subst(x^2+x+p, x, k))); \\ Michel Marcus, Jan 16 2015

CROSSREFS

Cf. A000040, A005846, A014556, A228123.

Sequence in context: A114215 A292302 A151712 * A186987 A300549 A038043

Adjacent sequences:  A253824 A253825 A253826 * A253828 A253829 A253830

KEYWORD

nonn

AUTHOR

Michel Lagneau, Jan 16 2015

STATUS

approved

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Last modified May 25 07:56 EDT 2020. Contains 334585 sequences. (Running on oeis4.)