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 A250040 Numbers n such that m = floor(n/10) is coprime to n and, if nonzero, m is also a term of the sequence. 9
 1, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 101, 103, 107, 109, 111, 112, 113, 114, 115, 116, 117, 118, 119, 121, 125, 127, 131, 132, 133, 134, 135, 136, 137, 138, 139, 141, 143, 145, 149, 151, 152, 154, 157, 158, 161, 163, 165, 167, 169, 171, 172, 173, 174, 175 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Equivalent definition 1: Assuming a base b (in this case b=10), let us say that a positive integer k has the property RTC(b) when m=floor(k/b) is coprime to k, i.e., gcd(k,m)=1. Then k belongs to this sorted list if (i) it has the property RTC(b) and (ii) m is either 0 or belongs also to the list. Equivalent definition 2: Every nonempty prefix of a(n) in base b has the property RTC(b). Notes: The acronym RTC stands for 'Right-Truncated is Coprime'. We could also say that a(n) are right-truncatable numbers with property RTC(b). This particular list is an infinite subset of A248499. LINKS Stanislav Sykora, Table of n, a(n) for n = 1..10000 Stanislav Sykora, PARI/GP scripts for genetic threads, with code and comments. Wikipedia, Coprime integers EXAMPLE 149, 14, and 1 are members because (149,14), (14,1) and (1,0) are all coprime pairs. 67 is not a member because gcd(67,7)=1, but gcd(6,0)=6. MAPLE F:= proc(a) seq(10*a+d, d = select(t -> igcd(a, t)=1, [\$0..9])) end proc: B:= : for i from 2 to 4 do   B[i]:= map(F, B[i-1]); od: ListTools:-Flatten([seq(B[i], i=1..4)]); # Robert Israel, Jan 04 2015 PROG (PARI) See the link. (PARI) is_rtc(n, b=10) = {while (((m=gcd(n\b, n)) == 1), if (m == 0, return (1)); if ((n=n\b) == 0, return (1)); ); return (0); } \\ Michel Marcus, Jan 17 2015 CROSSREFS Cf. A248499, A250041. Other lists of right-truncatable numbers with the property RTC(b): A250036 (b=4), A250038 (b=16), A250042 (b=9), A250044 (b=8), A250046 (b=7), A250048 (b=6), A250050 (b=5). Sequence in context: A131835 A262390 A102236 * A321485 A047842 A047843 Adjacent sequences:  A250037 A250038 A250039 * A250041 A250042 A250043 KEYWORD nonn,base AUTHOR Stanislav Sykora, Dec 07 2014 STATUS approved

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Last modified August 4 16:34 EDT 2020. Contains 336202 sequences. (Running on oeis4.)