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A159351 Smallest prime of the form a^2 + b^2 with 0 < a < b such that a + b = 2n+1. 1
5, 13, 29, 41, 61, 89, 113, 149, 181, 233, 269, 313, 389, 421, 521, 557, 613, 709, 761, 853, 929, 1013, 1109, 1201, 1301, 1409, 1553, 1637, 1741, 1861, 1997, 2113, 2269, 2381, 2521, 2677, 2837, 2969, 3121, 3461, 3449, 3613, 3797, 4001, 4153, 4337, 4513, 4729, 5081 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Conjecture: there is always such a prime number.

Primes of the form x^2 + y^2 with 0 < x < y such that there are no primes of the form (x+z)^2 + (y-z)^2 for 0 < z < (y-x)/2. Note: a(40) = 3461 > a(41) = 3449, so the order is not maintained. - Thomas Ordowski, Jan 21 2017

REFERENCES

L. E. Dickson, History of the Theory of Numbers, Vol, I: Divisibility and Primality, AMS Chelsea Publ., 1999

R. K. Guy, Unsolved Problems in Number Theory (2nd ed.) New York: Springer-Verlag, 1994

David Wells, Prime Numbers: The Most Mysterious Figures in Math. John Wiley and Sons. 2005

LINKS

Table of n, a(n) for n=1..49.

EXAMPLE

1^2 + 2^2 = 5 = a(1) = 1.

2^2 + 3^2 = 13 = a(2) < 1^2 + 4^2 = 17.

2^2 + 5^2 = 29 = a(3) < 1^2 + 6^2 = 37.

23^2 + 32^2 = 1553 = a(27) < 1597, 1657, 1693, 1733, 1777, 1877, 1933, 1993, 2273, 2437, 2617, 2713, 2917, 14 prime representations as sum of two squares.

PROG

(PARI) isok(p, n) = for (i=1, 2*n, if (i^2 + (2*n+1-i)^2 == p, return (1)); ); 0;

a(n) = {my(p = 2); while (! isok(p, n), p = nextprime(p+1)); p; } \\ Michel Marcus, Jan 29 2017

CROSSREFS

Cf. A145354, A159296, A157884, A264904.

Sequence in context: A158756 A185086 A277701 * A163251 A247177 A146286

Adjacent sequences:  A159348 A159349 A159350 * A159352 A159353 A159354

KEYWORD

nonn

AUTHOR

Ulrich Krug (leuchtfeuer37(AT)gmx.de), Apr 11 2009

EXTENSIONS

A-number in definition and cross-reference corrected, and more terms from R. J. Mathar, Apr 24 2009

Edited by Thomas Ordowski, Jan 25 2017

STATUS

approved

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Last modified January 24 10:24 EST 2020. Contains 331193 sequences. (Running on oeis4.)