A046429 Numbers requiring 9 steps to reach a prime under the prime factor concatenation procedure.

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

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 * A346804 A364997 A046796

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