A260597 Primes as they occur for the first time as factors of terms of A173426 = concatenation(1,2,...,n,n-1,...,1).
11, 3, 37, 101, 41, 271, 7, 13, 239, 4649, 73, 137, 333667, 12345678910987654321, 17636684157301569664903, 2799473675762179389994681, 1109, 4729, 2354041513534224607850261, 571, 3167, 10723, 439781, 2068140300159522133, 75401, 687437, 759077450603
Offset: 1
Keywords
Examples
A173426(1) = 1; A173426(2) = 121 = 11^2 => a(1) = 11. A173426(3) = 12321 = 3^2 37^2 => a(2..3) = (3, 37). A173426(4) = 1234321 = 11^2 101^2 => a(4) = 101. A173426(5) = 123454321 = 41^2 271^2 => a(5..6) = (41, 271). A173426(6) = 12345654321 = 3^2 7^2 11^2 13^2 37^2 => a(7..8) = (7, 13).
Links
- M. F. Hasler and Chai Wah Wu, Table of n, a(n) for n = 1..114 (a(n) for n = 1..84 from M. F. Hasler)
Programs
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PARI
S=[];apply(t->S=setunion(S,t=setminus(Set(t),S));t, vector(30,n,A260589_row(n)))
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Python
from sympy import primefactors A260597_list = [] for n in range(1,10): m = primefactors(int(''.join([str(d) for d in range(1,n+1)]+[str(d) for d in range(n-1,0,-1)]))) for p in m: if not p in A260597_list: A260597_list.append(p) # Chai Wah Wu, Aug 11 2015
Comments