

A273756


Least p for which min { x >= 0  p + (2n+1)*x + x^2 is composite } reaches the (local) maximum given in A273770.


4



41, 43, 47, 53, 61, 71, 83, 97, 113, 131, 151, 173, 197, 223, 251, 281, 313, 347, 383, 421, 461, 503, 547, 593, 641, 691, 743, 797, 73303, 73361, 73421, 73483, 3443897, 3071069, 3071137, 15949847, 76553693, 365462323, 365462399, 2204597, 9721, 1842719, 246407633, 246407719, 246407807, 246407897, 246407989
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OFFSET

0,1


COMMENTS

All terms are prime, since this is necessary and sufficient to get a prime for x = 0.
The values given in A273770 are the number of consecutive primes obtained for x = 0, 1, 2, ....
Sequence A273595 is the subsequence of terms for which 2n+1 is prime.
For even coefficients of the linear term, the answer would always be q=2, the only choice that yields a prime for x=0 and also for x=1 if (coefficient of the linear term)+3 is prime.
The initial term a(n=0) = 41 corresponds to Euler's famous primegenerating polynomial 41+x+x^2. Some subsequent terms are equal to the primes this polynomial takes for x=1,2,3,.... This stems from the fact that adding 2 to the coefficient of the linear term is equivalent to shifting the xvariable by 1. Since here we require x >= 0, we find a reduced subset of the previous sequence of primes, missing the first one, starting with q equal to the second one. (It is known that there is no better primegenerating polynomial of this form than Euler's, see the MathWorld page and A014556. "Better" means a larger p producing p1 primes in a row. However, the prime ktuple conjecture suggests that there should be arbitrarily long runs of primes of this form (for much larger p), i.e., longer than 41, but certainly much less than the respective p. Therefore we speak of local maxima.)


LINKS

Don Reble, Table of n, a(n) for n = 0..100
Eric Weisstein's World of Mathematics, PrimeGenerating Polynomial
Index to the OEIS, Entries related to primes produced by polynomials.


PROG

(PARI) A273756(n, p=2*n+1, L=10^(5+n\10), m=0, Q)={forprime(q=1, L, for(x=1, oo, ispseudoprime(q+p*x+x^2)&& next; x>m&& [Q=q, m=x]; break)); Q}


CROSSREFS

Cf. A273595, A273770.
Cf. also A002837 (n such that n^2n+41 is prime), A007634 (n such that n^2+n+41 is composite), A005846 (primes of form n^2+n+41), A097823, A144051, A187057 ... A187060, A190800, A191456 ff.
The first line of data coincides with that of A202018, A107448, A155884 (and also A140755, A142719, except for some initial terms), which are all related.
Sequence in context: A155884 A202018 A005846 * A154498 A223458 A243429
Adjacent sequences: A273753 A273754 A273755 * A273757 A273758 A273759


KEYWORD

nonn


AUTHOR

M. F. Hasler, May 26 2016


EXTENSIONS

Edited, following a remark by Don Reble, by M. F. Hasler, Jan 23 2018
a(27) corrected and more terms from Don Reble, Feb 15 2018


STATUS

approved



