A000203 -id:A000203 - cp's OEIS frontend

A144613 a(n) = sigma(3n) = A000203(3n).

4, 12, 13, 28, 24, 39, 32, 60, 40, 72, 48, 91, 56, 96, 78, 124, 72, 120, 80, 168, 104, 144, 96, 195, 124, 168, 121, 224, 120, 234, 128, 252, 156, 216, 192, 280, 152, 240, 182, 360, 168, 312, 176, 336, 240, 288, 192, 403, 228, 372, 234, 392, 216, 363, 288, 480, 260, 360

Offset: 1

N. J. A. Sloane, Jan 15 2009

Seiichi Manyama, Table of n, a(n) for n = 1..10000

Sigma(k*n): A000203 (k=1), A062731 (k=2), this sequence (k=3), A193553 (k=4), A283118 (k=5), A224613 (k=6), A283078 (k=7), A283122 (k=8), A283123 (k=9).

Cf. A008585, A078708.

a[n_] := DivisorSigma[1, 3*n]; Array[a, 60] (* Amiram Eldar, Dec 16 2022 *)

vector(66, n, sigma(3*n, 1)) \\ Joerg Arndt, Jul 30 2011

a(n) = A000203(n) + 3*A078708(n). - R. J. Mathar, May 19 2020

Sum_{k=1..n} a(k) = (11*Pi^2/36) * n^2 + O(n*log(n)). - Amiram Eldar, Dec 16 2022

Zero removed and offset corrected by Seiichi Manyama, Feb 28 2017

A211656 Numbers k such that the value of sigma(k) is unique; sigma(k) = A000203(k) = sum of divisors of k.

Original entry on oeis.org

1, 2, 3, 4, 5, 7, 8, 9, 12, 13, 18, 19, 22, 27, 29, 32, 36, 37, 43, 45, 49, 50, 61, 64, 67, 72, 73, 81, 91, 98, 100, 101, 106, 109, 121, 128, 129, 133, 134, 137, 146, 148, 149, 152, 157, 162, 163, 169, 171, 173, 192, 193, 197, 199, 200, 202, 211, 217, 218, 219

Offset: 1

Jaroslav Krizek, Apr 20 2012

nonn

Values of sigma(n) in increasing order are in A007370. Corresponding values of sigma(a(n)) is in A211657(n).
Complement of A206036 (numbers n such that sigma(n) = sigma(k) has solution for distinct numbers n and k).
Union of A066076 (primes p such that value of sigma(p) is unique) and A211658 (nonprimes p such that value of sigma(p) is unique).

			Number 36 is in sequence because sigma(36) = 91 and there is no other number m with sigma(m) = 91.
Number 6 is not in the sequence because sigma(6) = 12 and 12 is also sigma(11).

Robert Israel, Table of n, a(n) for n = 1..10000
Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems (invphi.gp).

Cf. A000203, A007370, A066076, A211657, A211658, A211659, A211660, A206036.

N:= 1000: # to get terms < the least m with sigma(m) > N
S:= map(numtheory:-sigma, [$1..N-1]):
m:=min(select(t -> S[t]>N, [$1..N-1]))-1:
select(n->numboccur(S[n],S)=1, [$1..m]); # Robert Israel, Jul 04 2019

nn = 300; mx = Max[DivisorSigma[1, Range[nn]]]; d = DivisorSigma[1, Range[mx]]; t = Transpose[Select[Sort[Tally[d]], #[[1]] <= mx && #[[2]] == 1 &]][[1]]; Select[Range[nn], MemberQ[t, d[[#]]] &] (* T. D. Noe, Apr 20 2012 *)

isok(k) = invsigmaNum(sigma(k)) == 1; \\ Amiram Eldar, Jan 11 2025, using Max Alekseyev's invphi.gp

A339106 Triangle read by rows: T(n,k) = A000203(n-k+1)*A000041(k-1), n >= 1, 1 <= k <= n.

Original entry on oeis.org

1, 3, 1, 4, 3, 2, 7, 4, 6, 3, 6, 7, 8, 9, 5, 12, 6, 14, 12, 15, 7, 8, 12, 12, 21, 20, 21, 11, 15, 8, 24, 18, 35, 28, 33, 15, 13, 15, 16, 36, 30, 49, 44, 45, 22, 18, 13, 30, 24, 60, 42, 77, 60, 66, 30, 12, 18, 26, 45, 40, 84, 66, 105, 88, 90, 42, 28, 12, 36, 39, 75, 56, 132, 90, 154, 120, 126, 56

Offset: 1

Omar E. Pol, Nov 23 2020

Conjecture 1: T(n,k) is the sum of all divisors of all (n - k + 1)'s in the n-th row of triangle A176206, assuming that A176206 has offset 1. The same for the triangle A340061.

Conjecture 2: the sum of row n equals A066186(n), the sum of all parts of all partitions of n.

			Triangle begins:
   1;
   3,  1;
   4,  3,  2;
   7,  4,  6,  3;
   6,  7,  8,  9,  5;
  12,  6, 14, 12, 15,  7;
   8, 12, 12, 21, 20, 21,  11;
  15,  8, 24, 18, 35, 28,  33,  15;
  13, 15, 16, 36, 30, 49,  44,  45,  22;
  18, 13, 30, 24, 60, 42,  77,  60,  66,  30;
  12, 18, 26, 45, 40, 84,  66, 105,  88,  90,  42;
  28, 12, 36, 39, 75, 56, 132,  90, 154, 120, 126, 56;
...
For n = 6 the calculation of every term of row 6 is as follows:
-------------------------
k   A000041        T(6,k)
1      1  *  12  =   12
2      1  *  6   =    6
3      2  *  7   =   14
4      3  *  4   =   12
5      5  *  3   =   15
6      7  *  1   =    7
.         A000203
-------------------------
The sum of row 6 is 12 + 6 + 14 + 12 + 15 + 7 = 66, equaling A066186(6).

Mirror of A221529.
Row sums give A066186 (conjectured).
Main diagonal gives A000041.
Columns 1 and 2 give A000203.
Column 3 gives A074400.
Column 4 gives A272027.
Column 5 gives A274535.
Column 6 gives A319527.
Cf. A176206, A340061.

T[n_, k_] := DivisorSigma[1, n - k + 1] * PartitionsP[k - 1]; Table[T[n, k], {n, 1, 12}, {k, 1, n}] // Flatten (* Amiram Eldar, Jan 08 2021 *)

T(n, k) = sigma(n-k+1)*numbpart(k-1); \\ Michel Marcus, Jan 08 2021

T(n,k) = sigma(n-k+1)*p(k-1), n >= 1, 1 <= k <= n.

A337209 Triangle read by rows T(n,k), (n >= 1, k > = 1), in which row n has length A000070(n-1) and every column gives A000203, the sum of divisors function.

Original entry on oeis.org

1, 3, 1, 4, 3, 1, 1, 7, 4, 3, 3, 1, 1, 1, 6, 7, 4, 4, 3, 3, 3, 1, 1, 1, 1, 1, 12, 6, 7, 7, 4, 4, 4, 3, 3, 3, 3, 3, 1, 1, 1, 1, 1, 1, 1, 8, 12, 6, 6, 7, 7, 7, 4, 4, 4, 4, 4, 3, 3, 3, 3, 3, 3, 3, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 15, 8, 12, 12, 6, 6, 6, 7, 7, 7, 7, 7, 4, 4, 4, 4, 4, 4, 4

Offset: 1

Omar E. Pol, Nov 27 2020

Conjecture: the sum of row n equals A066186(n), the sum of all parts of all partitions of n.

			Triangle begins:
   1;
   3,  1;
   4,  3, 1, 1;
   7,  4, 3, 3, 1, 1, 1;
   6,  7, 4, 4, 3, 3, 3, 1, 1, 1, 1, 1;
  12,  6, 7, 7, 4, 4, 4, 3, 3, 3, 3, 3, 1, 1, 1, 1, 1, 1, 1;
   8, 12, 6, 6, 7, 7, 7, 4, 4, 4, 4, 4, 3, 3, 3, 3, 3, 3, 3, 1, 1, 1, 1, 1, ...
  ...

Paolo Xausa, Table of n, a(n) for n = 1..10980 (rows 1..21 of the triangle, flattened)

Sum of divisors of terms of A176206.
Cf. A339278 (another version).
Cf. A000070, A000203, A066186, A221529, A238442, A339258.

A337209row[n_]:=Flatten[Table[ConstantArray[DivisorSigma[1,n-m],PartitionsP[m]],{m,0,n-1}]];Array[A337209row,10] (* Paolo Xausa, Sep 02 2023 *)

f(n) = sum(k=0, n-1, numbpart(k));
T(n, k) = {if (k > f(n), error("invalid k")); if (k==1, return (sigma(n))); my(s=0); while (k <= f(n-1), s++; n--;); sigma(1+s);}
tabf(nn) = {for (n=1, nn, for (k=1, f(n), print1(T(n,k), ", ");); );} \\ Michel Marcus, Jan 13 2021

T(n,k) = A000203(A176206(n,k)).

A033631 Numbers k such that sigma(phi(k)) = sigma(k) where sigma is the sum of divisors function A000203 and phi is the Euler totient function A000010.

Original entry on oeis.org

1, 87, 362, 1257, 1798, 5002, 9374, 21982, 22436, 25978, 35306, 38372, 41559, 50398, 51706, 53098, 53314, 56679, 65307, 68037, 89067, 108946, 116619, 124677, 131882, 136551, 136762, 138975, 144014, 160629, 165554, 170037, 186231, 192394, 197806

Offset: 1

Jud McCranie

nonn

For corresponding values of phi(k) and sigma(k), see A115619 and A115620.
This sequence is infinite because for each positive integer k, 3^k*7*1979 and 3^k*7*2699 are in the sequence (the proof is easy). A108510 gives primes p like 1979 and 2699 such that for each positive integer k, 3^k*7*p is in this sequence. - Farideh Firoozbakht, Jun 07 2005
There is another class of [conjecturally] infinite subsets connected to A005385 (safe primes). Examples: Let s,t be safe primes, s<>t, then 3^2*5*251*s, 2^2*61*71*s, 2*61*s*t and 2*19*311*s are in this sequence. So is 3*s*A108510(m). (There are some obvious exceptions for small s, t.) - Vim Wenders, Dec 27 2006

J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 87, p. 29, Ellipses, Paris 2008.
R. K. Guy, Unsolved Problems in Number Theory, B42.
D. Wells, The Penguin Dictionary of Curious and Interesting Numbers, Penguin Books 1997.
David Wells, Curious and Interesting Numbers (Revised), Penguin Books, page 114.

T. D. Noe, Table of n, a(n) for n = 1..1000
J.-M. De Koninck, F. Luca, Positive integers n such that sigma(phi(n))=sigma(n), JIS 11 (2008) 08.1.5.
S. W. Golomb, Equality among number-theoretic functions, Unpublished manuscript. (Annotated scanned copy)

Cf. A000203, A000010, A006872, A115619, A115620.

[k:k in [1..200000]| DivisorSigma(1,EulerPhi(k)) eq DivisorSigma(1,k)]; // Marius A. Burtea, Feb 09 2020

Do[If[DivisorSigma[1, EulerPhi[n]]==DivisorSigma[1, n], Print[n]], {n, 1, 10^5}]

is(n)=sigma(eulerphi(n))==sigma(n) \\ Charles R Greathouse IV, Feb 13 2013

Entry revised by N. J. A. Sloane, Apr 10 2006

A206032 a(n) = Product_{d|n} sigma(d) where sigma = A000203.

Original entry on oeis.org

1, 3, 4, 21, 6, 144, 8, 315, 52, 324, 12, 28224, 14, 576, 576, 9765, 18, 73008, 20, 95256, 1024, 1296, 24, 25401600, 186, 1764, 2080, 225792, 30, 26873856, 32, 615195, 2304, 2916, 2304, 1302170688, 38, 3600, 3136, 128595600, 42, 84934656, 44, 762048, 584064

Offset: 1

Jaroslav Krizek, Feb 03 2012

nonn

Sequence is not the same as A206031(n): a(66) = 429981696, A206031(66) = 35831808.

In sequence a(n) are multiplied all values of sigma(d) of all divisors d of numbers n, in sequence A206031 are multiplied only distinct values of sigma(d) of all divisors d of numbers n.

			For n=6 -> divisors d of 6: 1,2,3,6; corresponding values k of sigma(d): 1,3,4,12; a(6) = Product of k = 1*3*4*12 = 144. For n=66 -> divisors d of 66: 1,2,3,6,11,22,33,66; corresponding values k of sigma(d): 1,3,4,12,12,36,48,144; a(66) = Product of k = 1*3*4*12*12*36*48*144 = 429981696.

G. C. Greubel, Table of n, a(n) for n = 1..1000

Cf. A184388, A206031, A206033.

Table[Times @@ DivisorSigma[1, Divisors[n]], {n, 100}] (* T. D. Noe, Feb 10 2012 *)

a(n)=my(d=divisors(n));prod(i=2,#d,sigma(d[i])) \\ Charles R Greathouse IV, Feb 19 2013

a(p) = p+1, a(pq) = ((p+1)*(q+1))^2 for p, q = distinct primes.

A286357 One more than the exponent of the highest power of 2 dividing sigma(n): a(n) = A001511(A000203(n)).

Original entry on oeis.org

1, 1, 3, 1, 2, 3, 4, 1, 1, 2, 3, 3, 2, 4, 4, 1, 2, 1, 3, 2, 6, 3, 4, 3, 1, 2, 4, 4, 2, 4, 6, 1, 5, 2, 5, 1, 2, 3, 4, 2, 2, 6, 3, 3, 2, 4, 5, 3, 1, 1, 4, 2, 2, 4, 4, 4, 5, 2, 3, 4, 2, 6, 4, 1, 3, 5, 3, 2, 6, 5, 4, 1, 2, 2, 3, 3, 6, 4, 5, 2, 1, 2, 3, 6, 3, 3, 4, 3, 2, 2, 5, 4, 8, 5, 4, 3, 2, 1, 3, 1, 2, 4, 4, 2, 7, 2, 3, 4, 2, 4, 4, 4, 2, 5, 5, 2, 2, 3, 5, 4

Offset: 1

Antti Karttunen, May 10 2017

nonn

Antti Karttunen, Table of n, a(n) for n = 1..10000
Index entries for sequences related to sigma(n)

Cf. A000203, A000523, A001511, A053866, A070939, A082903, A161942, A286358.

Table[IntegerExponent[DivisorSigma[1,n],2]+1,{n,120}] (* Harvey P. Dale, Sep 04 2023 *)

A001511(n) = (1+valuation(n,2));
A286357(n) = A001511(sigma(n));
for(n=1, 10000, write("b286357.txt", n, " ", A286357(n)));

from sympy import divisor_sigma as D
def a001511(n): return bin(n)[2:][::-1].index("1") + 1
def a(n): return a001511(D(n)) # Indranil Ghosh, May 12 2017

from sympy import divisor_sigma
def A286357(n): return ((m:=int(divisor_sigma(n)))&-m).bit_length() # Chai Wah Wu, Jul 10 2022

(define (A286357 n) (A001511 (A000203 n)))
(define (A286357 n) (A070939 (/ (A000203 n) (A161942 n))))

a(n) = A001511(A000203(n)).
a(n) = 1 + A000523(A000203(n)/A161942(n)). [See also A082903.]
a(n) = 1 iff A053866(n) = 1.

A286360 Compound filter (prime signature & sum of the divisors): a(n) = P(A046523(n), A000203(n)), where P(n,k) is sequence A000027 used as a pairing function.

Original entry on oeis.org

1, 8, 12, 49, 23, 142, 38, 239, 124, 259, 80, 753, 107, 412, 412, 1051, 173, 1237, 212, 1390, 672, 826, 302, 3427, 565, 1087, 1089, 2223, 467, 5080, 530, 4403, 1384, 1717, 1384, 7911, 743, 2086, 1836, 6352, 905, 7780, 992, 4477, 3928, 2932, 1178, 14583, 1774, 5368, 2932, 5898, 1487, 10177, 2932, 10177, 3576, 4471, 1832, 25711, 1955, 5056, 6567, 18019, 3922

Offset: 1

Antti Karttunen, May 10 2017

nonn

Antti Karttunen, Table of n, a(n) for n = 1..10000
Eric Weisstein's World of Mathematics, Pairing Function
Index entries for sequences related to sigma(n)

Cf. A000027, A286359, A286460.

Cf. A007503, A065608 (sequences matching to this filter), also A000203, A046523, A161942, A286034, A286357.

A000203(n) = sigma(n);
A046523(n) = { my(f=vecsort(factor(n)[, 2], , 4), p); prod(i=1, #f, (p=nextprime(p+1))^f[i]); };  \\ This function from Charles R Greathouse IV, Aug 17 2011
A286360(n) = (1/2)*(2 + ((A046523(n)+A000203(n))^2) - A046523(n) - 3*A000203(n));
for(n=1, 10000, write("b286360.txt", n, " ", A286360(n)));

from sympy import factorint, divisor_sigma as D
def T(n, m): return ((n + m)**2 - n - 3*m + 2)/2
def P(n):
    f = factorint(n)
    return sorted([f[i] for i in f])
def a046523(n):
    x=1
    while True:
        if P(n) == P(x): return x
        else: x+=1
def a(n): return T(a046523(n), D(n)) # Indranil Ghosh, May 12 2017

(define (A286360 n) (* (/ 1 2) (+ (expt (+ (A046523 n) (A000203 n)) 2) (- (A046523 n)) (- (* 3 (A000203 n))) 2)))

a(n) = (1/2)*(2 + ((A046523(n)+A000203(n))^2) - A046523(n) - 3*A000203(n)).

A295296 Numbers n such that the sum of their divisors + the number of ones in their binary expansion = 2n; numbers for which A000203(n) + A000120(n) = 2n.

Original entry on oeis.org

1, 2, 3, 4, 8, 10, 16, 32, 64, 128, 136, 256, 315, 512, 1024, 2048, 4096, 8192, 16384, 32768, 32896, 65536, 131072, 262144, 524288, 1048576, 2097152, 4194304, 8388608, 16777216, 33554432, 67108864, 134217728, 268435456, 536870912, 1073741824, 2147483648, 2147516416

Offset: 1

Antti Karttunen, Nov 26 2017

Numbers n such that their binary weight is equal to their deficiency.
Numbers n such that A000120(n) = A033879(n), or equally A000203(n) = A005187(n), or equally A001065(n) = A011371(n).
2^(2^k-1) * (2^(2^k) + 1) is in the sequence if and only if (2^(2^k) + 1) is a (Fermat) prime (A019434) which as of today is known to be the case for 0 <= k <= 4 giving the terms 3, 10, 136, 32896 and 2147516416. - David A. Corneth, Nov 26 2017
It would be nice to know whether 315 is the only term that is neither in A191363 nor a power of two.
Any term that is either a square or twice a square (in A028982) must be odious (in A000069), and vice versa.
If there's an odd term below 10^30 besides 315 then it must be divisible by a prime >= 23. - David A. Corneth, Nov 27 2017
221753180448460815 is odd and also a term of this sequence. - Alexander Violette, Dec 24 2020

			A000203(315) = 1 + 3 + 5 + 7 + 9 + 15 + 21 + 35 + 45 + 63 + 105 + 315 = 624. 2*315 - 624 = 6, and when 315 is written in binary, 100111011, we see that it has six 1-bits. Thus 315 is included in the sequence.

Max Alekseyev, Table of n, a(n) for n = 1..70 (first 52 terms from Giovanni Resta)
Index entries for sequences related to binary expansion of n
Index entries for sequences related to sigma(n)

Positions of zeros in A294898 and A294899.
Subsequence of A005100 and also of A295298.
Subsequences: A000079, A191363 (the five known terms).
Cf. A000120, A000203, A001065, A005187, A011371, A019434, A033879, A141548.

Select[Range[2^20], DivisorSigma[1, #] + DigitCount[#, 2, 1] == 2 # &] (* Michael De Vlieger, Nov 26 2017 *)

for(n=1, oo, if(sigma(n)+hammingweight(n) == 2*n, print1(n, ", ")));

Terms a(35) and beyond from Giovanni Resta, Feb 27 2020

A318468 a(n) = 2*n AND A000203(n), where AND is bitwise-and (A004198) and A000203 = sum of divisors.

Original entry on oeis.org

0, 0, 4, 0, 2, 12, 8, 0, 0, 16, 4, 24, 10, 24, 24, 0, 2, 36, 4, 40, 32, 36, 8, 48, 18, 32, 32, 56, 26, 8, 32, 0, 0, 4, 0, 72, 2, 12, 8, 80, 2, 64, 4, 80, 74, 72, 16, 96, 32, 68, 64, 96, 34, 104, 72, 112, 80, 80, 52, 40, 58, 96, 104, 0, 0, 128, 4, 8, 0, 128, 8, 128, 2, 16, 20, 136, 0, 136, 16, 160, 32, 36, 4, 160, 40, 132, 40, 176, 18, 160, 48

Offset: 1

Antti Karttunen, Aug 26 2018

Cf. A000203, A003987, A318466, A318467.

Cf. also A318458.

Array[BitAnd[2 #, DivisorSigma[1, #]] &, 91] (* Michael De Vlieger, Apr 21 2019 *)

PARI
```
A318468(n) = bitand(2*n,sigma(n));
```

a(n) = A004198(2*n, A000203(n)).

a(n) = A224880(n) - A318466(n) = (A224880(n)-A318467(n))/2.

cp's OEIS Frontend

A144613 a(n) = sigma(3*n) = A000203(3*n).

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A211656 Numbers k such that the value of sigma(k) is unique; sigma(k) = A000203(k) = sum of divisors of k.

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A339106 Triangle read by rows: T(n,k) = A000203(n-k+1)*A000041(k-1), n >= 1, 1 <= k <= n.

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A337209 Triangle read by rows T(n,k), (n >= 1, k > = 1), in which row n has length A000070(n-1) and every column gives A000203, the sum of divisors function.

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A033631 Numbers k such that sigma(phi(k)) = sigma(k) where sigma is the sum of divisors function A000203 and phi is the Euler totient function A000010.

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Magma

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A206032 a(n) = Product_{d|n} sigma(d) where sigma = A000203.

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A286357 One more than the exponent of the highest power of 2 dividing sigma(n): a(n) = A001511(A000203(n)).

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A144613 a(n) = sigma(3n) = A000203(3n).