A390629 Smallest number k >= 1 starting a run of n numbers i = k..k + n - 1 for which i and its digit-reverse are not coprime, while k+n and its digit-reverse are coprime.

9, 8, 7, 6, 5, 4, 3, 2, 2318, 22179, 22178, 2153825, 2153824, 2153823, 2153822, 21234885, 21234884, 2548954051, 2548954050, 2827029885, 2827029884, 22196843221, 22196843220, 271884060481, 271884060480

Offset

1,1

Links

Table of n, a(n) for n=1..25.

Example

a(2) = 8 since gcd(8,8) = 8, gcd(9,9) = 9 and gcd(10,1) = 1; no smaller number works.

Prog

(Python)
from math import gcd
from itertools import count, islice
def c(i): return gcd(i, int(str(i)[::-1])) > 1
def A390629_gen(startn=1, startk=1): # generator of terms
    adict, n, r = dict(), startn, 1
    for k in count(startk):
        if c(k): r += 1
        else:
            for v in range(r-1, 0, -1):
                if v not in adict:
                    adict[v] = k - v
            while n in adict: yield adict[n]; n += 1
            r = 1
print(list(islice(A390629_gen(), 17))) # Michael S. Branicky, Nov 14 2025

Crossrefs

Cf. A000040, A004086, A069554.

Sequence in context: A342115 A090671 A132673 * A363249 A107927 A019890

Adjacent sequences: A390626 A390627 A390628 * A390630 A390631 A390632

Keyword

nonn,base,more

Author

Jean-Marc Rebert, Nov 13 2025

Extensions

a(18)-a(25) from Michael S. Branicky, Nov 14 2025

Status

approved