A351239 -id:A351239 - cp's OEIS frontend

A116436 Numbers m which when sandwiched between two 1's give a multiple of m.

1, 11, 13, 77, 91, 137, 9091, 909091, 5882353, 10989011, 12987013, 52631579, 76923077, 90909091, 4347826087, 9090909091, 13698630137, 909090909091, 3448275862069, 10989010989011, 12987012987013, 76923076923077, 90909090909091, 9090909090909091, 909090909090909091

Offset: 1

Giovanni Resta, Feb 15 2006

All k-digit numbers that divide 10^{k+1} + 1. - Franklin T. Adams-Watters, Apr 23 2008
Notice the infinite pattern m = (90..90..90)91 with 1m1/m = 21, e.g., 1911/91 = 190911/9091 = 19090911/909091 = 21 (see A095372). - Zak Seidov, Apr 22 2008
Corresponding numbers k such that k * a(n) = 1.a(n).1 where '.' stands for concatenation are in A351320. - Bernard Schott, Feb 07 2022

			77 is a member since 1771 is a multiple of 77 (77*23).

Michael S. Branicky, Table of n, a(n) for n = 1..1441 (terms 1..87 from Franklin T. Adams-Watters)

Subsequence of A116437, A116438, A116439, A116440, A116441, A116442, A116443, A116444.
Cf. A136296, A329914, A329915, A351320.
Some subsequences, M such that k*M=1M1 for: A095372 \ {1} (k=21), A331630 (k=23), A351237 (k=83), A351238 (k=87), A351239 (k=101).

f[k_, d_] := Flatten@Table[Select[Divisors[k*(10^(i + 1) + 1)], IntegerLength[ # ] == i &], {i, d}]; f[1, 14] (* Ray Chandler, May 11 2007 *)

A116436(k) = {local(l, d, lb, ub); d=divisors(10^(k+1)+1);l=[];lb=10^(k-1); ub=10*lb; for(i=1,#d, if(d[i]>=lb&&d[i]A116436(i))); l
\\ Franklin T. Adams-Watters, Apr 22 2008

from sympy import isprime
from itertools import count, islice
def agen(): # generator of terms
    yield 1
    for k in count(2):
        t = 10**(k+1) + 1
        yield from (t//i for i in range(100, 10, -1) if t%i == 0)
print(list(islice(agen(), 25))) # Michael S. Branicky, Mar 26 2023 following Franklin T. Adams-Watters but removing factorization

A351320(n) * a(n) = 1.a(n).1 where "." stands for concatenation. - Bernard Schott, Feb 07 2022

A329915 a(n) is the least M such that A329914(n) * M = 1M1, where 1M1 denotes the concatenation of 1, M and 1.

Original entry on oeis.org

91, 77, 5882353, 52631579, 4347826087, 3448275862069, 2127659574468085106383, 20408163265306122449, 1694915254237288135593220339, 16393442622950819672131147541, 137, 13, 112359550561797732809, 11, 10309278350515463917525773195876288659793814433

Offset: 1

Bernard Schott, Nov 24 2019

When M is a q-digit term, then M is a divisor of 10^(q+1) + 1.
For each term k in A329914, there exist a set of numbers M_k which, when 1 is placed at both ends of M_k, the number M_k is multiplied by k. This sequence gives the smallest integer M(k) = M of each set {M_k}.
See A329914 for further information about these numbers.

			A329914(1) = 21 and 21 * 91 = 1[91]1, and there is no integer < 91 that satisfies this relation, so a(1) = 91.
A329914(2) = 23 and 23 * 77 = 1[77]1, and there is no integer < 77 that satisfies this relation, so a(2) = 77.
A329914(5) = 33 and 33 * 4347826087 = 1[4347826087]1, and there is no integer < 4347826087 that satisfies this relation, so a(5) = 4347826087.

D. Wells, 112359550561797732809 entry, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986, Revised edition 1997, p. 196.

Cf. A000533, A329914 (corresponding numbers k).

Some corresponding sets {M_k} : A095372 \ {1} = {M_21}, A331630 = {M_23}, A351237 = {M_83}, A351238 = {M_87}, A351239 = {M_101}.

A351237 Numbers M such that 83 * M = 1M1, where 1M1 denotes the concatenation of 1, M and 1.

Original entry on oeis.org

137, 13698630137, 1369863013698630137, 136986301369863013698630137, 13698630136986301369863013698630137, 1369863013698630136986301369863013698630137, 136986301369863013698630136986301369863013698630137

Offset: 1

Bernard Schott, Feb 05 2022

There are only 15 numbers k such that there exist numbers M_k which, when 1 is placed at both ends of M_k, the number M_k is multiplied by k; 83 is the eleventh such integer, so 83 = A329914(11), and a(1) = A329915(11) = 137; hence, the terms of this sequence form the infinite set {M_83}.

Every term M = a(n) has q = 8*n-5 digits, and 10^(q+1)+1 that has q = 8*n-5 zeros in its decimal expansion is equal to 73 * M, so a(n) = M is a divisor of 10^(8*n-4)+1. Example: a(2) = 13698630137 has 11 digits and 73 * 13698630137 = 1000000000001 that has 11 zeros in its decimal expansion.

			83 * 137 = 1[137]1, hence 137 is a term.
83 * 13698630137 = 1[13698630137]1, and 13698630137 is another term.

D. Wells, 112359550561797732809 entry, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986, Revised edition 1997, p. 196.

Index entries for linear recurrences with constant coefficients, signature (100000001,-100000000).

Subsequence of A116436.
Cf. A329914, A329915.
Similar for: A095372 \ {1} (k=21), A331630 (k=23), this sequence (k=83), A351238 (k=87), A351239 (k=101).

Maple
```
seq((10^(8*n-4)+1)/73, n=1..15);
```

Table[(10^(8*n-4)+1)/73, {n, 1, 7}] (* Amiram Eldar, Feb 06 2022 *)
LinearRecurrence[{100000001,-100000000},{137,13698630137},20] (* Harvey P. Dale, Nov 01 2022 *)

a(n) = (10^(8*n-4)+1)/73 for n >= 1.

A351238 Numbers M such that 87 * M = 1M1, where 1M1 denotes the concatenation of 1, M and 1.

Original entry on oeis.org

13, 12987013, 12987012987013, 12987012987012987013, 12987012987012987012987013, 12987012987012987012987012987013, 12987012987012987012987012987012987013, 12987012987012987012987012987012987012987013, 12987012987012987012987012987012987012987012987013, 12987012987012987012987012987012987012987012987012987013

Offset: 1

Bernard Schott, Feb 05 2022

There are only 15 numbers k such that there exist numbers M_k which, when 1 is placed at both ends of M_k, the number M_k is multiplied by k; 87 is the twelfth such integer, so 87 = A329914(12), and a(1) = A329915(12) = 13; hence, the terms of this sequence form the infinite set {M_87}.

Every term M = a(n) has q = 6*n-4 digits, and 10^(q+1)+1 that has q = 6*n-4 zeros in its decimal expansion is equal to 77 * M, so a(n) = M is a divisor of 10^(6*n-3)+1. Example: a(2) = 12987013 has 8 digits and 77 * 12987013 = 1000000001 that has 8 zeros in its decimal expansion.

			87 * 13 = 1[13]1, hence 13 is a term.
87 * 12987013 = 1[12987013]1, and 12987013 is a term.

D. Wells, 112359550561797732809 entry, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986, Revised edition 1997, p. 196.

Index entries for linear recurrences with constant coefficients, signature (1000001,-1000000).

Subsequence of A116436.
Cf. A329914, A329915.
Similar for: A095372 \ {1} (k=21), A331630 (k=23), A351237 (k=83), this sequence (k=87), A351239 (k=101).

Maple
```
seq((10^(6*n-3)+1)/77, n=1..15);
```

Table[(10^(6*n - 3) + 1)/77, {n, 1, 10}] (* Amiram Eldar, Feb 06 2022 *)

a(n) = (10^(6*n-3)+1)/77 for n >= 1.

A351320 a(n) is the unique integer k such that k * A116436(n) = 1.A116436(n).1 where "." stands for concatenation.

Original entry on oeis.org

111, 101, 87, 23, 21, 83, 21, 21, 27, 101, 87, 29, 23, 21, 33, 21, 83, 21, 39, 101, 87, 23, 21, 21, 21, 83, 101, 87, 59, 23, 21, 99, 57, 21, 27, 21, 101, 87, 29, 23, 21, 83, 69, 21, 71, 21, 101, 87, 33, 23, 21, 21, 83, 21, 101, 87, 23, 21, 27, 21, 39, 21, 83, 101, 87, 29, 23, 21, 21, 107, 21, 101

Offset: 1

Bernard Schott, Feb 07 2022

Except for a(1) = 111, which is unique, all terms appear infinitely many times and belong to this set of fifteen integers: {21, 23, 27, 29, 33, 39, 57, 59, 69, 71, 83, 87, 99, 101, 107}; see A329914.

The corresponding indices where these integers appear the first time are respectively: 5, 4, 9, 12, 15, 19, 33, 29, 43, 45, 6, 3, 32, 2, 70.

			A116436(1) = 1 and 111 * 1 = 1.1.1, hence a(1) = 111.
A116436(2) = 11 and 101 * 11 = 1.11.1, hence a(2) = 101.
A116436(32) = 112359550561797752809 and 99 * 112359550561797752809 = 1.112359550561797752809.1 hence a(32) = 99 (see Penguin reference).

David Wells, 112359550561797732809 entry, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986, Revised edition 1997, p. 196.

Cf. A116436, A329914, A329915.

M such that k*M=1M1 for: A095372 \ {1} (k=21), A331630 (k=23), A351237 (k=83), A351238 (k=87), A351239 (k=101).

A116436(k) = {local(l, d, lb, ub); d=divisors(10^(k+1)+1); l=[]; lb=10^(k-1); ub=10*lb; for(i=1, #d, if(d[i]>=lb&&d[i]A116436
a(n) = {my(v6=[], i=1); while (#v6 < n, v6 = concat(v6, A116436(i)); i++); my(x= v6[n]); my(k=1); while (eval(Str(1, x, 1)) % x, k++); eval(Str(1, x, 1))/x;} \\ Michel Marcus, Feb 10 2022

a(n) * A116436(n) = 1.A116436(n).1 where "." stands for concatenation.

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A116436 Numbers m which when sandwiched between two 1's give a multiple of m.

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A329915 a(n) is the least M such that A329914(n) * M = 1M1, where 1M1 denotes the concatenation of 1, M and 1.

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A351237 Numbers M such that 83 * M = 1M1, where 1M1 denotes the concatenation of 1, M and 1.

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A351238 Numbers M such that 87 * M = 1M1, where 1M1 denotes the concatenation of 1, M and 1.

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A351320 a(n) is the unique integer k such that k * A116436(n) = 1.A116436(n).1 where "." stands for concatenation.

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