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Is there a natural number $k\ge 1$, such that $11^k+k^{11}$ is prime ?

I checked the numbers upto $k=3000$ and did not find a prime number. On the other hand, for $k=76$ and for $k=142$, there is no small prime factor (http://factordb.com/index.php?query=11%5En%2Bn%5E11&use=n&n=1&sent=Show&VP=on&VC=on&EV=on&FF=on&perpage=200&format=1)

I cannot find a reason that $11^k+k^{11}$ must be composite for every $k\ge 2$ , for example because we can show that algebraic or aurifeuillian factors must exist.

Peter
  • 86,576

1 Answers1

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You did not check quite far enough. $11^{3100}+3100^{11}$

$=2076462013023723087177998371272629862972883202603058638957413939624626$ $9054240698423859652025721171495403183350925185327797086492200207876487$
$2657520023839950330031205883217443601268748782180924528196277373392564$ $4013266704424018036520898747850451175736951267420442305614435735217327$ $3971093935910922095895742597356204629264981808290204113439085537450904$ $1054716017041784246181255107634302837222895447128850530244058819200215$ $2362870017852948178955739528730187590107749130966131061785154952403600$ $7845259931210631021751416234425093201119837728222208324398548386042895$ $9584011040615951740645304512954981656788544230076265875001026816088705$ $6757307230421011598858076933236873935084068306578841134872968149221788$ $0717817364570019141856814901370572392334191752979013227566442886023434$ $7285704153224120229496485704857078154855202352223685401328549708656537$ $2745103719682773351988518415795763107623953506997522673140901829546037$ $8418905329436977478581214760667145942730920343954357670045645928260538$ $8061193029697759164275948364125097680392267910933560309185294496717695$ $7869168427818687080167508439109316989952426208387505064118843391126904$ $8809439180832931991818599346591305891117152763850592098966904948668260$ $7218165257463715929191901113894143802681224509003171286959752701316000$ $5149727764856832739587273643660076429162547313056786072738282051427143$ $9059263411928545834661840831401401793837448294722205096917466151658118$ $6345840545618942101785072630580361383262366193852415870364775543675397$ $7411556835071609937251107708659627426204781277098083989903899975463323$ $0297607332321477653359454339442543504332822307845301255056579651182760$ $3197214280255284847766302464422208909240777526693592272352267547214798$ $9279732186573642952954772173255469021780823659279533971856573468351741$ $3270025882653830663768988149700654876357099836728713717899352117384639$ $9993052359435350597385181227038675446499316142028048344457869772495345$ $1159392734366193006180448767048576470894500056348333701493332319434778$ $5490882030798588653090632863574646380731305190374645481940398446527831$ $0722028247156052753014565407431192576032277984102207926118128755743971$ $8340985455265496911157467144349144537796113479118340686160573603130766$ $1099123424689008589232889055800413648775020397984208864809200556571931$ $1582800146997406161685727978841949295312774927206609369501218544956948$ $8530144248346132022932824472115888270376191468004515998634183646374938$ $7492291650568266714709826604252169619563731647361927100184078247062181$ $5224715926448818703439624866795396110172267449029716976203305026056481$ $8323355342007447572624200323791062810049504835193102542786506344964036$ $2891135921476558034980600392420027840933546664159778908938853495165590$ $8907646835427289592872012218998684082574118686855754880914643032478309$ $8873209783047376068101435630510253377633655787909767034270013933181425$ $6575659583293821949031203842568947299487797641091314053619475074233370$ $0097144671013963445110078855965695128408676810683471873791915379119568$ $5384648967264244558381359201014768419939135316262258913110928604355249$ $1071911512953583221678783961802928432334347315692012480310793727126598$ $7925284867474264761232617448873485010382295276804354264892976999063391$ $0697911628932721420433293413759170876405267637504799091260822608897310$ $259826001$

is prime.

almagest
  • 18,650
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    Thank you for the search. I continued and came across this number. Did you proof that the number is prime , or is it "just" a probable-prime ? – Peter Jun 22 '16 at 15:04
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    http://math.stackexchange.com/questions/123465/do-we-really-know-the-reliability-of-primeqn-for-n1016 – almagest Jun 22 '16 at 15:06
  • Unlike you, large numbers are not my field. It is a while since I looked at the latest tests proving primality with certainty, but I suspect proving with certainty would be quite hard. – almagest Jun 22 '16 at 15:11
  • That is true. The number has $3,229$ digits , so a rigorous proof will take a while. But the number passes many SPRP-tests, so it is without doubt prime. It is curious, that the number of digits is prime as well. – Peter Jun 22 '16 at 15:14
  • Ok, you got me hooked. How is that number certified as prime? – Oscar Lanzi Jun 22 '16 at 19:08
  • primo can do a primilaty-proof. I know two methods : The adleman-pomerance-rumely-test (APR) and the elliptic-curve-prime-proving-method (ECPP). I do not know which method is used by primo (perhaps none of the two). The calculation will probably take several hours, possibly even several days. But since the number passes more than 300 SPRP-tests, the probability that the number is composite, is so small, that a proof does not really pay off. – Peter Jun 24 '16 at 13:45
  • You missed the golden opportunity to add '…, obviously.' – LSpice Dec 14 '19 at 03:40