OFFSET
0,2
FORMULA
a(n) = 10 + n for 0 < n < 10.
a(n) <= concat(1,n) with equality when n is prime.
EXAMPLE
a(3) = a(1*3) = 13 and similar for 1 <= n <= 9, cf. first formula.
a(11) = a(1*11) = 111, a(13) = a(1*13) = 113, a(17) = a(1*17) = 117 etc. according to the second formula with prime n.
a(10) = a(2*5) = 25, a(14) = a(2*7) = 27, a(15) = a(3*5) = 35 etc. for semiprime indices; in these cases a(p*q) = concat(p,q) where p is the lexicographic smaller factor, but this is not always the case.
a(22) = 122, not concat(11,2), although 11*2 = 22, but the smallest product that can be formed by slicing 112 in two parts is A347470(112) = 1*12 = 12, less than 22.
a(93) = a(3*31) = concat(93,1) because concat(1,93) gives 19*3, concat(3,31) gives 33*1 and concat(31,3) gives 3*13 as smaller products.
PROG
CROSSREFS
KEYWORD
nonn,base
AUTHOR
M. F. Hasler, Sep 03 2021
STATUS
approved