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 A075023 a(n) = the smallest prime divisor of A173426(n) = concatenation of (1, 2, 3,..., n, n-1, ..., 1) for n > 1; a(1) = 1. 7
 1, 11, 3, 11, 41, 3, 239, 11, 3, 12345678910987654321, 7, 3, 1109, 7, 3, 71, 7, 3, 251, 7, 3, 70607, 7, 3, 989931671244066864878631629, 7, 3, 149, 7, 3, 827, 7, 3, 197, 7, 3, 39907897297, 7, 3, 17047, 7, 3, 191, 7, 3, 967, 7, 3, 139121, 7, 3, 109, 7, 3, 5333, 7, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS FORMULA a(n) = A020639(A173426(n)). a(3n) = 3 for all n > 0. a(3n-1) = 7 for 3 < n < 34. - M. F. Hasler, Jul 29 2015 EXAMPLE a(5) = 41 as 123454321 = 41*41*271*271. a(25) = 989931671244066864878631629 is the smaller factor of the semiprime A173426(24) = a(25) * A075023(25). A173426(37) = 39907897297 * P58 * P59, where Pxx are primes with xx digits, therefore a(37) = 39907897297. PROG (PARI) A075023(n)=A020639(A173426(n)) \\ Efficient code for computing the least prime factor should be developed in A020639. For n = 37, use \g3 (debugging level 3) to see the lpf within milliseconds, while factorization would take hours. - M. F. Hasler, Jul 29 2015 CROSSREFS Cf. A075019, A075020, A075021, A075022, A075024. Sequence in context: A110774 A261411 A067063 * A205960 A302556 A088262 Adjacent sequences:  A075020 A075021 A075022 * A075024 A075025 A075026 KEYWORD base,nonn AUTHOR Amarnath Murthy, Sep 01 2002 EXTENSIONS More terms from Sascha Kurz, Jan 03 2003 Terms beyond a(24) from M. F. Hasler, Jul 29 2015 STATUS approved

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Last modified August 17 17:08 EDT 2019. Contains 326059 sequences. (Running on oeis4.)