OFFSET
1,8
COMMENTS
a(n) = 0 if n is already prime. a(n) = -1 for n = any multiple of 7 other than 7 itself. Each multiple of 7 has been tested out to 2000 7's with no result found. The first eight values of n which are not multiples of 7 for which no answer has yet been found are 95, 480, 851, 891, 957, 1184, 1261, 1881. 95 has been tested out to 2100 7's, 1881 has been tested out to 1750 7's, the others have been tested out to 2000 7's. Pending solutions for these values of n, the first 10 record holders are currently 1, 8, 20, 40, 120, 128, 225, 260, 296, 711 with the values 1, 2, 6, 12, 16, 18, 56, 182, 434, 1648 respectively.
From Toshitaka Suzuki, Sep 26 2023: (Start)
a(95) = 2904, a(480) = 11330, a(851) = 28895, a(891) = -1, a(957) = 2903, a(1184) = 4646, a(1261) = -1 and a(1881) = 47927.
a(n) = -1 when n = 15873*k + 891, 1261, 2889, 3263, 3300, 7810, 8917, 9812, 12617, 13024, 14615 or 15066, because n followed by any positive number, m say, of 7's is divisible by at least one of the primes {3,11,13,37}.
Similarly,
a(n) = -1 when n = 11111111*k + 964146, 1207525, 2342974, 3567630, 7525789, 8134540, 8591231 or 9641467 by primes {11,73,101,137};
a(n) = -1 when n = 429000429*k + 23928593, 27079312, 36492115, 41207969, 52285750, 80569929, 89920882, 93857078, 133928703, 217208145, 223492302, 236849444, 239285937, 247857232, 259793116, 270793127, 323985244, 332698824, 333570182, 334985255, 346849554, 364921157, 376698868 or 412079697 by primes {3,11,13,101,9901};
a(n) = -1 when n = 1221001221*k + 14569863, 28792885, 145698637, 167698659, 225079510, 235985156, 247079532, 287928857, 331921124, 399492478, 415286113, 421492500, 437286135, 455985376, 489857474, 529929099, 551921344, 635208563, 709857694, 877208805, 896850104, 993570842, 1029793886 or 1138850346 by primes {3,11,37,101,9901};
a(n) = -1 when n = 1443001443*k + 85928655, 167698659, 176928746, 218921011, 233985154, 247079532, 310492389, 326286024, 376857361, 585793442, 655208583, 700699192, 746208674, 780080065, 791570640, 805850013, 843492922, 859286557, 882570731, 896850104, 1027793884, 1219922012, 1234986155 or 1377858362 by primes {3,13,37,101,9901}.
The first 10 record holders are 1, 8, 20, 40, 95, 480, 851, 1881, 2038, 2174 with the values 1, 2, 6, 12, 2904, 11330, 28895, 47927, 76206, 94146 respectively.
a(4444) > 300000 or a(4444) = -1.
(End)
For multiples of 7 it is confusing that the author writes in the first comment "has been tested out to 2000": If we denote n{m} = n*10^m + (10^m-1)/9*7 the number n with m '7's appended, then it is easy to see that (7k){m} / 7 = k*10^m + (10^m-1)/9 is an integer for all m >= 0. - M. F. Hasler, Jun 05 2024
LINKS
Toshitaka Suzuki, Table of n, a(n) for n = 1..4443
EXAMPLE
a(20) = 6 because six 7's must be appended to 20 before a prime is formed (20777777).
a(14) = -1 because no matter how many 7's are appended to 14, the resulting number is always divisible by 7 and can therefore not be prime.
PROG
(PARI) apply( {A090464(n, LIM=500)=n%7 && for(m=0, LIM, ispseudoprime(n) && return(m); n=n*10+7); -(n>7)}, [1..55]) \\ Retun value -1 means that a(n) = -1 or, if n%7 > 0, then possibly a(n) > LIM, the search limit given as 2nd (optional) parameter. - M. F. Hasler, Jun 05 2024
CROSSREFS
KEYWORD
base,sign
AUTHOR
Chuck Seggelin (barkeep(AT)plastereddragon.com), Dec 02 2003
STATUS
approved