OFFSET
1,1
COMMENTS
It is conjectured (but still an open problem) that there exist infinitely many primes of the form n^2 + n + 1 = ((2*n+1)^2 + 3)/4.
Landau's 4th problem from (1912, 5th Congress of Mathematicians in Cambridge) conjectures that there are infinitely many primes of the form n^2 + 1 (also Euler 1760; Mirsky 1949).
Hardy and Littlewood proposed a conjecture about the asymptotic number of primes of the form n^2 + 1.
An emirp ("prime" spelled backwards) is a prime whose reversal is a different prime, the reversal of q is denoted by R(q).
It is conjectured but also unproved that there are infinitely many emirps (see A048054).
For p > 3 necessarily p of the form 6*k + 5 as (6*k+1)^2 + (6*k+1) + 1 a multiple of 3.
REFERENCES
M. Gardner: Die magischen Zahlen des Dr. Matrix, Krueger Verlag, Frankfurt am Main, 1987
R. Guy: Unsolved Problems in Number Theory,3rd edition, Springer, New York, 2004
G. H. Hardy, E. M. Wright: Einfuehrung in die Zahlentheorie, R. Oldenburg, Muenchen, 1958
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
3^2 + 3 + 1 = 13 = prime(6), R(13) = prime(11), 3 is first term.
5^2 + 5 + 1 = 31 = prime(11), R(31) = prime(6), 5 is 2nd term.
q = 1811^2 + 1811 + 1 = 3281533 = prime(235691), R(q) = prime(240351), first case that p = 1811 = prime(280) = emirp(87) is itself an emirp.
MAPLE
filter:= proc(p) local q, qr;
if not isprime(p) then return false fi;
q:= p^2+p+1;
if not isprime(q) then return false fi;
qr:= revdigs(q);
qr <> q and isprime(qr);
end proc:
select(filter, [3, seq(i, i=5..50000, 6)]); # Robert Israel, Dec 04 2016
MATHEMATICA
EmirpQ[n_] := If[ PrimeQ@n, Block[{id = IntegerDigits@n}, rid = Reverse@ id; rid != id && PrimeQ@ FromDigits@ rid]]; Select[ Prime@ Range@ 3500, EmirpQ[ #^2 + # + 1] &] (* Robert G. Wilson v, Jul 26 2010 *)
CROSSREFS
KEYWORD
AUTHOR
Ulrich Krug (leuchtfeuer37(AT)gmx.de), May 29 2010
EXTENSIONS
More terms from Robert G. Wilson v, Jul 26 2010
STATUS
approved