login
A046429
Numbers requiring 9 steps to reach a prime under the prime factor concatenation procedure.
1
40, 44, 81, 224, 265, 395, 422, 462, 640, 698, 818, 972, 1010, 1032, 1070, 1089, 1174, 1206, 1280, 1336, 1446, 1518, 1520, 1528, 1581, 1662, 1728, 1814, 1816, 1849, 1852, 1853, 1856, 1892, 1927, 1932, 1960, 2032, 2060, 2061, 2090, 2098, 2202, 2212, 2249
OFFSET
1,1
LINKS
Patrick De Geest, Home Primes
EXAMPLE
698 is in the sequence as 698 -> 2349 -> 333329 -> 2571297 -> 3857099 -> 31312323 -> 33771937101 -> 379437170413 -> 73124171910091 -> 374148203145623. Only after the ninth iteration we reach a prime. - David A. Corneth, Oct 15 2019
PROG
(PARI) is(n, k) = if(isprime(n), return(0)); for(i = 1, k - 1, n = concatelements(primesvector(n)); if(isprime(n), return(0))); n = concatelements(primesvector(n)); isprime(n)
concatelements(v) = my(s = ""); for(i = 1, #v, s = concat(s, v[i])); eval(s)
primesvector(n) = my(f = factor(n), res = vector(vecsum(f[, 2])), t = 0); for(i = 1, #f~, for(j = 1, f[i, 2], t++; res[t] = f[i, 1])); res \\ David A. Corneth, Oct 15 2019
CROSSREFS
Sequence in context: A095513 A095505 A095497 * A346804 A364997 A046796
KEYWORD
nonn,base
AUTHOR
Patrick De Geest, Jun 15 1998
EXTENSIONS
Extended and edited by Charles R Greathouse IV, Apr 28 2010
STATUS
approved