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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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

David A. Corneth, Table of n, a(n) for n = 1..10000

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

Cf. A037271, A046411.

Sequence in context: A095513 A095505 A095497 * A046796 A333712 A204746

Adjacent sequences:  A046426 A046427 A046428 * A046430 A046431 A046432

KEYWORD

nonn,base

AUTHOR

Patrick De Geest, Jun 15 1998

EXTENSIONS

Extended and edited by Charles R Greathouse IV, Apr 28 2010

STATUS

approved

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Last modified August 1 14:44 EDT 2021. Contains 346393 sequences. (Running on oeis4.)