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A162489 Least y such that x^y + y^x is prime, for x = A162488(n). 4
2, 2, 2, 2, 5, 15, 2, 33, 7, 3, 21, 8, 34, 9, 80, 56, 67, 9, 32, 65, 45, 133, 98, 36, 51, 157, 76, 214, 200, 87, 91, 111, 122, 342, 20, 142, 364, 289, 9, 184, 98, 423, 365, 20, 56, 441, 329, 8, 234, 234, 157, 291, 91, 379, 98, 464, 518, 325, 32, 654, 87, 634, 34, 21, 443 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Sequences A162488 and A162490 list the corresponding x values and primes.

See there and the main entry A094133 for more information, links and references.

LINKS

Table of n, a(n) for n=1..65.

FORMULA

a(n)^A162488(n)+A162488(n)^a(n) = A162490(n)

EXAMPLE

The least x such that x^y + y^x is prime for some x>y>1 is A162488(1)=3, the smallest such y is a(1)=2, yielding the prime A162490(1) = 9 + 8 = 17.

MATHEMATICA

lst = {}; Do[ If[ PrimeQ[x^y + y^x], AppendTo[lst, {x, y}]], {x, 3, 750}, {y, 2, x - 1}]; Transpose[ lst][[2]] (* Robert G. Wilson v, Aug 17 2009 *)

PROG

(PARI) for(i=3, 999, for(j=2, i-1, isprime(i^j+j^i)|next; print1(j", "); break))

CROSSREFS

Cf. A094133, A162486 - A162490.

Sequence in context: A222255 A126788 A098789 * A079894 A292586 A324291

Adjacent sequences:  A162486 A162487 A162488 * A162490 A162491 A162492

KEYWORD

nonn

AUTHOR

M. F. Hasler, Jul 04 2009

EXTENSIONS

More terms from Robert G. Wilson v, Aug 17 2009

STATUS

approved

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Last modified April 19 08:42 EDT 2019. Contains 322241 sequences. (Running on oeis4.)