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 A102751 Numbers n such that 1+(n-1)^2 and ((n-1)/2)^2+((n+1)/2)^2=(1/2)*(n^2+1) are primes. 0
 3, 5, 11, 15, 25, 85, 95, 121, 131, 171, 181, 205, 231, 261, 271, 315, 441, 445, 471, 545, 571, 595, 715, 751, 781, 861, 921, 951, 1011, 1055, 1081, 1095, 1125, 1151, 1185, 1315, 1411, 1421, 1495, 1615, 1661, 1701, 2035, 2051, 2055, 2065, 2175, 2261, 2315 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Conjectured to be infinite. We still do not know if there are an infinite number of primes of the form n^2 + 1, which is Landau's 4th Problem, declared "unattackable" in 1912. See Hardy and Wright; Ribenboim. - Jonathan Vos Post, Feb 28 2005 REFERENCES Hardy, G. H. and Wright, W. M., Unsolved Problems Concerning Primes, Section 2.8 and Appendix 3 in An Introduction to the Theory of Numbers, 5th ed. Oxford, England: Oxford University Press, p. 19. Ribenboim, P. The New Book of Prime Number Records, 3rd ed. New York: Springer-Verlag, pp. 206-208, 1996. LINKS Eric Weisstein's World of Mathematics, Landau's Problems. EXAMPLE a(3)=11 because 10^2+1=101 and 5^2+6^2=(1/2)*(11^2+1)=61 are prime MAPLE a:=proc(n) if isprime(1+(n-1)^2)=true and type((n^2+1)/2, integer)=true and isprime((n^2+1)/2)=true then n else fi end: seq(a(n), n=1..3000); # Emeric Deutsch, May 31 2005 CROSSREFS Cf. A036468, A002496, A005574. Sequence in context: A136977 A003529 A018667 * A032673 A164053 A200176 Adjacent sequences:  A102748 A102749 A102750 * A102752 A102753 A102754 KEYWORD nonn AUTHOR Robin Garcia, Feb 09 2005 EXTENSIONS More terms from Emeric Deutsch, May 31 2005 STATUS approved

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Last modified April 8 14:52 EDT 2020. Contains 333314 sequences. (Running on oeis4.)