prog.sf

#!/usr/bin/ruby

# a(1) = 1; for n>1, a(n) = the largest prime divisor of the number C(n) formed from the concatenation of 1,2,3,... up to n.
# https://oeis.org/A075022

# First several terms of the sequence:
#   1, 3, 41, 617, 823, 643, 9721, 14593, 3803, 1234567891, 630803, 2110805449, 869211457, 205761315168520219, 8230452606740808761, 1231026625769, 584538396786764503, 801309546900123763, 833929457045867563

# a(n) = A006530(A007908(n)). - ~~~~

# (PARI) a(n) = if(n==1, 1, vecmax(factor(eval(concat(apply(k->Str(k), [1..n]))))[, 1])); \\ ~~~~

include("../../../factordb/auto.sf")

func a(n) {
    1..n -> to_a.join.to_i
}

var bfile = File("bfile.txt").open_w.autoflush(true)

for n in (1..10000) {
    var row = "#{n} #{gpf(a(n))||1}"
    say row
    bfile.say(row)
}