cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

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A324320 Terms of A324315 (squarefree integers m > 1 such that if prime p divides m, then the sum of the base p digits of m is at least p) that are also octagonal numbers (A000567) with index equal to their largest prime factor.

Original entry on oeis.org

1045, 2465, 2821, 15841, 20501, 34133, 51221, 68101, 89441, 116033, 118405, 162401, 170885, 216545, 300833, 364705, 439301, 472033, 530881, 642181, 687365, 746005, 970145, 976981, 997633, 1104133, 1148245, 1193221, 1231361, 1239061, 1398101, 1654661, 1971541
Offset: 1

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2465 is also a Carmichael number (A002997).
2821 is also a primary Carmichael number (A324316).
See the section on polygonal numbers in Kellner and Sondow 2019.
Subsequence of the special polygonal numbers A324973. - Jonathan Sondow, Mar 27 2019

Examples

			A324315(4) = 1045 = 5 * 11 * 19 = 19 * (3 * 19 - 2) = A000567(19), so 1045 is a member.
		

Crossrefs

Programs

  • Mathematica
    SD[n_, p_] := If[n < 1 || p < 2, 0, Plus @@ IntegerDigits[n, p]];
    LP[n_] := Transpose[FactorInteger[n]][[1]];
    ON[n_] := n(3n - 2);
    TestS[n_] := (n > 1) && SquareFreeQ[n] && VectorQ[LP[n], SD[n, #] >= # &];
    Select[ON@ Prime[Range[100]], TestS[#] &]

Extensions

More terms from Amiram Eldar, Dec 05 2020

A324404 Squarefree integers m > 1 such that if prime p divides m, then s_p(m) >= p and s_p(m) == 2 (mod p-1), where s_p(m) is the sum of the base p digits of m.

Original entry on oeis.org

1122, 3458, 5642, 6734, 11102, 13202, 17390, 17822, 21170, 22610, 27962, 31682, 46002, 58682, 61778, 79730, 82082, 93314, 105266, 106262, 125490, 127946, 136202, 150722, 153254, 177122, 182002, 202202, 203870, 214370, 231842, 252434, 274298, 278462, 305102, 315282
Offset: 1

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Comments

For d >= 1 define S_d = (terms m in A324315 such that s_p(m) == d (mod p-1) if prime p divides m). Then S_1 is precisely the Carmichael numbers (A002997), S_2 is A324404, S_3 is A324405, and the union of all S_d for d >= 1 is A324315.
Subsequence of the 2-Knödel numbers (A050990). Generally, for d > 1 the terms of S_d that are greater than d form a subsequence of the d-Knödel numbers.
See Kellner and Sondow 2019.

Examples

			1122 = 2*3*11*17 is squarefree and equals 10001100010_2, 1112120_3, 930_11, and 3f0_17 in base p = 2, 3, 11, and 17. Then s_2(1122) = 1+1+1+1 = 4 >= 2, s_3(1122) = 1+1+1+2+1+2 = 8 >= 3, s_11(1122) = 9+3 = 12 >= 11, and s_17(1122) = 3+f = 3+15 = 18 >= 17. Also, s_2(1122) = 4 == 2 (mod 1), s_3(1122) = 8 == 2 (mod 2), s_11(1122) = 12 == 2 (mod 10), and s_17(1122) = 18 == 2 (mod 16), so 1122 is a member.
		

Crossrefs

Programs

  • Mathematica
    SD[n_, p_] := If[n < 1 || p < 2, 0, Plus @@ IntegerDigits[n, p]];
    LP[n_] := Transpose[FactorInteger[n]][[1]];
    TestSd[n_, d_] := (n > 1) && (d > 0) && SquareFreeQ[n] && VectorQ[LP[n], SD[n, #] >= # && Mod[SD[n, #] - d, # - 1] == 0 &];
    Select[Range[200000], TestSd[#, 2] &]

Extensions

More terms from Amiram Eldar, Dec 05 2020

A324405 Squarefree integers m > 1 such that if prime p divides m, then s_p(m) >= p and s_p(m) == 3 (mod p-1), where s_p(m) is the sum of the base p digits of m.

Original entry on oeis.org

3003, 3315, 5187, 7395, 8463, 14763, 19803, 26733, 31755, 47523, 50963, 58035, 62403, 88023, 105339, 106113, 123123, 139971, 152643, 157899, 166611, 178923, 183183, 191919
Offset: 1

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Comments

For d >= 1 define S_d = (terms m in A324315 such that s_p(m) == d (mod p-1) if prime p divides m). Then S_1 is precisely the Carmichael numbers (A002997), S_2 is A324404, S_3 is A324405, and the union of all S_d for d >= 1 is A324315.
Subsequence of the 3-Knödel numbers (A033553). Generally, for d > 1 the terms of S_d that are greater than d form a subsequence of the d-Knödel numbers.
See Kellner and Sondow 2019.

Examples

			3003 = 3*7*11*13 is squarefree and equals 11010020_3, 11520_7, 2290_11, and 14a0_13 in base p = 3, 7, 11, and 13. Then s_3(3003) = 1+1+1+2 = 5 >= 3, s_7(3003) = 1+1+5+2 = 9 >= 7, s_11(3003) = 2+2+9 = 13 >= 11, and s_13(3003) = 1+4+a = 1+4+10 = 15 >= 13. Also, s_3(3003) = 5 == 3 (mod 2), s_7(3003) = 9 == 3 (mod 6), s_11(3003) = 13 == 3 (mod 10), and s_13(3003) = 15 == 3 (mod 12), so 3003 is a member.
		

Crossrefs

Programs

  • Mathematica
    SD[n_, p_] := If[n < 1 || p < 2, 0, Plus @@ IntegerDigits[n, p]];
    LP[n_] := Transpose[FactorInteger[n]][[1]];
    TestSd[n_, d_] := (n > 1) && (d > 0) && SquareFreeQ[n] && VectorQ[LP[n], SD[n, #] >= # && Mod[SD[n, #] - d, # - 1] == 0 &];
    Select[Range[200000], TestSd[#, 3] &]

A324318 Number of terms in A324315 (squarefree integers m > 1 such that if prime p divides m, then the sum of the base p digits of m is at least p) less than 10^n.

Original entry on oeis.org

0, 0, 2, 57, 636, 7048, 75150, 801931, 8350039, 86361487
Offset: 1

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The number of squarefree integers less than 10^n is 0, 6, 61, 608, 6083, 60794, 607926, 6079291, 60792694, 607927124, ... (see A053462).

Examples

			There are two terms of A324315 less than 10^3, namely, 231 and 561, so a(3) = 2.
		

Crossrefs

A324973 Special polygonal numbers.

Original entry on oeis.org

6, 15, 66, 70, 91, 190, 231, 435, 561, 703, 715, 782, 861, 946, 1045, 1105, 1426, 1653, 1729, 1770, 1785, 1794, 1891, 2035, 2278, 2465, 2701, 2821, 2926, 3059, 3290, 3367, 3486, 3655, 4371, 4641, 4830, 5005, 5083, 5151, 5365, 5551, 5565, 5995, 6441, 6545, 6601
Offset: 1

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Squarefree polygonal numbers P(r,p) = (p^2*(r-2)-p*(r-4))/2 whose greatest prime factor is p >= 3, and whose rank (or order) is r >= 3 (see A324974).
The Carmichael numbers A002997 and primary Carmichael numbers A324316 are subsequences. See Kellner and Sondow 2019.

Examples

			P(3,5) = 15 is squarefree, and its greatest prime factor is 5, so 15 is a member.
More generally, if p is an odd prime and P(3,p) is squarefree, then P(3,p) is a member, since P(3,p) = (p^2+p)/2 = p*(p+1)/2, so p is its greatest prime factor.
CAUTION: P(6,7) = 91 = 7*13 is a member even though 7 is NOT its greatest prime factor, as P(6,7) = P(3,13) and 13 is its greatest prime factor.
		

Crossrefs

Subsequence of A324972 = intersection of A005117 and A090466.
A002997, A324316, A324319 and A324320 are subsequences.

Programs

  • Mathematica
    GPF[n_] := Last[Select[Divisors[n], PrimeQ]];
    T = Select[Flatten[Table[{p, (p^2*(r - 2) - p*(r - 4))/2}, {p, 3, 150}, {r, 3, 100}], 1], SquareFreeQ[Last[#]] && First[#] == GPF[Last[#]] &];
    Take[Union[Table[Last[t], {t, T}]], 47]
  • PARI
    is(k) = if(issquarefree(k) && k>1, my(p=vecmax(factor(k)[, 1]), r); p>2 && (r=2*(k/p-1)/(p-1)) && denominator(r)==1, 0); \\ Jinyuan Wang, Feb 18 2021

Extensions

Several missing terms inserted by Jinyuan Wang, Feb 18 2021

A324456 Numbers m > 1 such that there exists a divisor g > 1 of m which satisfies s_g(m) = g.

Original entry on oeis.org

6, 10, 12, 15, 18, 20, 21, 24, 28, 33, 34, 36, 39, 40, 45, 48, 52, 57, 63, 65, 66, 68, 72, 76, 80, 85, 87, 88, 91, 93, 96, 99, 100, 105, 111, 112, 117, 120, 126, 130, 132, 133, 135, 136, 144, 145, 148, 153, 156, 160, 165, 171, 175, 176, 185, 186, 189, 190
Offset: 1

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Author

Bernd C. Kellner, Feb 28 2019

Keywords

Comments

The function s_g(m) gives the sum of the base-g digits of m.
The sequence is infinite, since it contains A324460.
The sequence also contains the 3-Carmichael numbers A087788 and the primary Carmichael numbers A324316.
A term m must have at least 2 prime factors, and the divisor g satisfies the inequalities 1 < g < m^(1/(ord_g(m)+1)) <= sqrt(m), where ord_g(m) gives the maximum exponent e such that g^e divides m.
Note that the sequence contains the 3-Carmichael numbers, but not all Carmichael numbers. This is a nontrivial fact.
The subsequence A324460 mainly gives examples in which g is composite.
See Kellner 2019.
It appears that g is usually prime: compare with A324857 (g prime) and the sparser sequence A324858 (g composite). However, g is usually composite for higher values of m. - Jonathan Sondow, Mar 17 2019

Examples

			6 is a member, since 2 divides 6 and s_2(6) = 2.
		

Crossrefs

Subsequences are A033502, A087788, A324316, A324458, A324460.
Subsequence of A324455.
Union of A324857 and A324858.

Programs

  • Mathematica
    s[n_, g_] := If[n < 1 || g < 2, 0, Plus @@ IntegerDigits[n, g]];
    f[n_] := AnyTrue[Divisors[n], s[n, #] == # &];
    Select[Range[5000], f[#] &]
  • PARI
    isok(n) = {fordiv(n, d, if ((d>1) && (sumdigits(n, d) == d), return (1)););} \\ Michel Marcus, Mar 19 2019

A324460 Numbers m > 1 that have a strict s-decomposition.

Original entry on oeis.org

45, 96, 225, 325, 405, 576, 637, 640, 891, 1225, 1377, 1408, 1536, 1701, 1729, 2025, 2541, 2821, 3321, 3751, 3825, 4225, 4608, 4961, 6400, 6517, 6525, 7381, 7840, 8125, 8281, 9216, 9537, 9801, 10625, 10935, 12025, 12288, 12825, 12936, 13125, 13312, 13357
Offset: 1

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Author

Bernd C. Kellner, Feb 28 2019

Keywords

Comments

The sequence contains the primary Carmichael numbers A324316.
The sequence is infinite. If f(x) counts such numbers m below x, then f(x) > 1/11 x^(1/3) - 1/3 for x >= 1.
A number m > 1 has a strict s-decomposition if there exists a decomposition in n proper factors g_k with exponents e_k >= 1 (the factors g_k being strictly increasing but not necessarily coprime) such that
m = g_1^e_1 * ... * g_n^e_n, where s_{g_k}(m) = g_k for all k,
and s_g(m) gives the sum of the base-g digits of m.
A term m has the following properties:
m must have at least 2 factors g_k. If m = g_1^e_1 * g_2^e_2 with exactly two factors, then e_1 + e_2 >= 3.
Each factor g_k of m satisfies the inequalities 1 < g_k < m^(1/(ord_{g_k}(m)+1)) <= sqrt(m), where ord_g(m) gives the maximum exponent e such that g^e divides m.
See Kellner 2019.

Examples

			Since 576 = 2^4 * 6^2 with s_2(576) = 2 and s_6(576) = 6, 576 is a member.
		

Crossrefs

Subsequences are A324316, A324458. Subsequence of A324459.

Programs

  • Mathematica
    s[n_, g_] := If[n < 1 || g < 2, 0, Plus @@ IntegerDigits[n, g]];
    HasDecompS[m_] := Module[{E0, EV, G, R, k, n, v},
    If[m < 1 || !CompositeQ[m], Return[False]];
    G = Select[Divisors[m], s[m, #] == # &];
    n = Length[G]; If[n < 2, Return[False]];
    E0 = Array[0 &, n]; EV = Array[v, n];
    R = Solve[Product[G[[k]]^EV[[k]], {k, 1, n}] == m && EV >= E0, EV, Integers]; Return[R != {}]];
    Select[Range[10^4], HasDecompS[#] &]

A324455 Numbers m > 1 such that there exists a divisor g > 1 of m which satisfies s_g(m) >= g.

Original entry on oeis.org

6, 10, 12, 14, 15, 18, 20, 21, 22, 24, 26, 28, 30, 33, 34, 36, 38, 39, 40, 42, 44, 45, 46, 48, 50, 51, 52, 54, 56, 57, 58, 60, 62, 63, 65, 66, 68, 69, 70, 72, 74, 75, 76, 78, 80, 82, 84, 85, 86, 87, 88, 90, 91, 92, 93, 94, 95, 96, 98, 99, 100, 102, 104, 105
Offset: 1

Views

Author

Bernd C. Kellner, Feb 28 2019

Keywords

Comments

The function s_g(m) gives the sum of the base-g digits of m.
The sequence is infinite, since it contains A324460 and the Carmichael numbers A002997.
A term m must have at least 2 prime factors, and the divisor g satisfies the inequalities 1 < g < m^(1/(ord_g(m)+1)) <= sqrt(m), where ord_g(m) gives the maximum exponent e such that g^e divides m.
See Kellner 2019.

Examples

			6 is a member, since 2 divides 6 and s_2(6) = 2.
		

Crossrefs

Programs

  • Mathematica
    s[n_, g_] := If[n < 1 || g < 2, 0, Plus @@ IntegerDigits[n, g]];
    f[n_] := AnyTrue[Divisors[n], s[n, #] >= # &];
    Select[Range[1000], f[#] &]

A324457 Numbers m > 1 such that every prime divisor p of m satisfies s_p(m) >= p.

Original entry on oeis.org

24, 45, 48, 72, 96, 120, 144, 189, 192, 216, 224, 225, 231, 240, 288, 315, 320, 325, 336, 352, 360, 384, 405, 432, 450, 480, 525, 540, 560, 561, 567, 576, 594, 600, 637, 648, 672, 704, 720, 768, 792, 819, 825, 832, 850, 864, 891, 896, 924, 945, 960, 975, 980
Offset: 1

Views

Author

Bernd C. Kellner, Feb 28 2019

Keywords

Comments

The function s_p(m) gives the sum of the base-p digits of m.
The sequence is infinite, since it contains A324315, and thus the Carmichael numbers A002997.
Being a subsequence of A324459, a term m has the following properties:
m must have at least 2 prime factors. If m = p1^e1 * p2^e2 with two primes p1 and p2, then e1 + e2 >= 3.
Each prime factor p of m satisfies the inequalities p < m^(1/(ord_p(m)+1)) <= sqrt(m), where ord_p(m) gives the maximum exponent e such that p^e divides m.
In the terminology of A324459, the prime factorization of m equals an s-decomposition of m.
See Kellner 2019.
a(n) is a Carmichael number A002997 iff a(n) is squarefree and s_p(a(n)) == 1 (mod p-1) for every prime factor p of a(n). See Kellner and Sondow 2019. - Jonathan Sondow, Mar 16 2019

Examples

			The number 45 has the prime factors 3 and 5. Since s_3(45) = 3 and s_5(45) = 5, 45 is a member.
		

Crossrefs

Subsequences are A002997, A324315, and A324458.
Subsequence of A324459 and A324857.

Programs

  • Mathematica
    s[n_, p_] := If[n < 1 || p < 2, 0, Plus @@ IntegerDigits[n, p]];
    f[n_] := AllTrue[Transpose[FactorInteger[n]][[1]], s[n, #] >= # &];
    Select[Range[10^4], f[#] &]

A324458 Numbers m > 1 such that every prime divisor p of m satisfies s_p(m) = p.

Original entry on oeis.org

45, 325, 405, 637, 891, 1729, 2821, 3751, 4961, 6517, 7381, 8125, 8281, 10625, 13357, 21141, 26353, 28033, 29341, 31213, 33125, 35443, 46657, 47081, 58621, 65341, 74431, 78625, 81289, 94501, 98125, 99937, 123823, 146461, 231601, 236321, 252601, 254221, 294409
Offset: 1

Views

Author

Bernd C. Kellner, Feb 28 2019

Keywords

Comments

The function s_p(m) gives the sum of the base-p digits of m.
The sequence contains the primary Carmichael numbers A324316.
Being a subsequence of A324460, a term m has the following properties:
m must have at least 2 prime factors. If m = p1^e1 * p2^e2 with two primes p1 and p2, then e1 + e2 >= 3.
Each prime factor p of m satisfies the inequalities p < m^(1/(ord_p(m)+1)) <= sqrt(m), where ord_p(m) gives the maximum exponent e such that p^e divides m.
In the terminology of A324460, the prime factorization of m equals a strict s-decomposition of m.
See Kellner 2019.
a(n) is squarefree iff a(n) is a primary Carmichael number A324316. - Jonathan Sondow, Mar 16 2019

Examples

			The number 45 has the prime factors 3 and 5. Since s_3(45) = 3 and s_5(45) = 5, 45 is a member.
		

Crossrefs

Subsequence is A324316. Subsequence of A324457, A324459, and A324460.

Programs

  • Mathematica
    s[n_, p_] := If[n < 1 || p < 2, 0, Plus @@ IntegerDigits[n, p]];
    f[n_] := AllTrue[Transpose[FactorInteger[n]][[1]], s[n, #] == # &];
    Select[Range[10^7], f[#] &]
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