A057950 -id:A057950 - cp's OEIS frontend

A054520 Let S = {1,5,9,13,..., 4n+1, ...} and call p in S an S-prime if p>1 and the only divisors of p in S are 1 and p; sequence gives elements of S that are not S-primes.

1, 25, 45, 65, 81, 85, 105, 117, 125, 145, 153, 165, 169, 185, 189, 205, 221, 225, 245, 261, 265, 273, 285, 289, 297, 305, 325, 333, 345, 357, 365, 369, 377, 385, 405, 425, 429, 441, 445, 465, 477, 481, 485, 493, 505, 513, 525, 533, 545, 549, 561, 565, 585

Offset: 1

N. J. A. Sloane, Apr 09 2000

The set S is a standard example of a set where unique factorization does not hold.

With the exception a(1)=1, numbers of the form 4*(m + n + 4*m*n)+1 (m,n>0). No such number can be prime because 4*(m + n + 4*m*n)+1=(4m+1)*(4n+1). - Artur Jasinski, Sep 22 2008

			49 is an S-prime.

T. M. Apostol, Introduction to Analytic Number Theory, Springer-Verlag, page 101, problem 1.

William A. Tedeschi, Table of n, a(n) for n = 1..10000
Eric Weisstein's World of Mathematics, Hilbert Number

Cf. A057948, A057949, A057950.

a = {}; Do[Do[AppendTo[a, 4(m + n + 4 m n)+1], {m, 1, 100}], {n, 1, 100}]; Union[a] (* Artur Jasinski, Sep 22 2008 *)

ok(n)={if(n%4==1, my(f=factor(n)); 2<>sum(i=1, #f~, f[i,2]*if(f[i,1]%4==3, 1, 2)), 0)} \\ Andrew Howroyd, Nov 25 2018

More terms from James Sellers, Apr 11 2000

Offset corrected by Andrew Howroyd, Nov 25 2018

A057948 S-primes: let S = {1,5,9, ... 4i+1, ...}; then an S-prime is in S but is not divisible by any members of S except itself and 1.

Original entry on oeis.org

5, 9, 13, 17, 21, 29, 33, 37, 41, 49, 53, 57, 61, 69, 73, 77, 89, 93, 97, 101, 109, 113, 121, 129, 133, 137, 141, 149, 157, 161, 173, 177, 181, 193, 197, 201, 209, 213, 217, 229, 233, 237, 241, 249, 253, 257, 269, 277, 281, 293, 301, 309, 313, 317, 321, 329

Offset: 1

Jud McCranie, Oct 14 2000

nonn

Factorization in S is not unique. See related sequences.
Kostrikin calls these numbers quasi-primes. - Arkadiusz Wesolowski, Aug 19 2017
a(n) is a prime of the form 4*n + 1 or a product of 2 primes of the form 4*n + 3. - David A. Corneth, Nov 10 2018

			21 is of the form 4i+1, but it is not divisible by any smaller S-primes, so 21 is in the sequence.

T. M. Apostol, Introduction to Analytic Number Theory, Springer-Verlag, page 101, problem 1.
A. I. Kostrikin, Introduction to Algebra, universitext, Springer, 1982.

Robert Israel, Table of n, a(n) for n = 1..10000
Eric Weisstein's World of Mathematics, Hilbert Number [From _Eric W. Weisstein_, Sep 15 2008]

Union of A002144 and A107978. - Charlie Neder, Nov 03 2018

Cf. A054520, A057949, A057950.

N:= 1000: # to get all terms <= N
S:= {seq(4*i+1,i=1..floor((N-1)/4))}:
for n from 1 while n <= nops(S) do
  r:= S[n];
  S:= S minus {seq(i*r,i=2..floor(N/r))};
od:
S; # Robert Israel, Dec 14 2014

nn = 100; Complement[Table[4 k + 1, {k, 1, nn}], Union[Flatten[ Table[Table[(4 k + 1) (4 j + 1), {k, 1, j}], {j, 1, nn}]]]] (* Geoffrey Critzer, Dec 14 2014 *)

is(n) = if(n % 2 == 0, return(0)); if(n%4 == 1 && isprime(n), return(1)); f = factor(n); if(vecsum(f[, 2]) != 2, return(0)); for(i = 1, #f[, 1], if(f[i, 1] % 4 == 1, return(0))); n>1 \\ David A. Corneth, Nov 10 2018

a(n) ~ C n log n / log log n, where C > 2. - Thomas Ordowski, Sep 09 2012

Offset corrected by Charlie Neder, Nov 03 2018

A057949 Numbers with more than one factorization into S-primes. See A054520 and A057948 for definition.

Original entry on oeis.org

441, 693, 1089, 1197, 1449, 1617, 1881, 1953, 2205, 2277, 2541, 2709, 2793, 2961, 3069, 3249, 3381, 3465, 3717, 3933, 3969, 4221, 4257, 4389, 4473, 4557, 4653, 4761, 4977, 5229, 5301, 5313, 5445, 5733, 5841, 5929, 5985, 6237, 6321, 6417, 6489, 6633

Offset: 1

Jud McCranie, Oct 14 2000

nonn

Numbers with k >= 4 prime factors (with multiplicity) that are congruent to 3 mod 4, no k-1 of which are equal. - Charlie Neder, Nov 03 2018

			2205 is in S = {1,5,9, ... 4i+1, ...}, 2205 = 5*9*49 = 5*21^2; 5, 9, 21 and 49 are S-primes (A057948).

Eric M. Schmidt, Table of n, a(n) for n = 1..10000

Cf. A054520, A057948, A057950.

Cf. A343826 (only 1 way), A343827 (exactly 2 ways), A343828 (exactly 3 ways).

ok(n)={if(n%4==1, my(f=factor(n)); my(s=[f[i,2] | i<-[1..#f~], f[i,1]%4==3]); vecsum(s)>=4 && vecmax(s)Andrew Howroyd, Nov 25 2018

def A057949_list(bound) :
    numterms = (bound-1)//4 + 1
    M = [1] * numterms
    for k in range(1, numterms) :
        if M[k] == 1 :
            kpower = k
            while kpower < numterms :
                step = 4*kpower+1
                for j in range(kpower, numterms, step) :
                    M[j] *= 4*k+1
                kpower = 4*kpower*k + kpower + k
    # Now M[k] contains the product of the terms p^e where p is an S-prime
    # and e is maximal such that p^e divides 4*k+1
    return [4*k+1 for k in range(numterms) if M[k] > 4*k+1]
# Eric M. Schmidt, Dec 11 2016

Offset corrected by Eric M. Schmidt, Dec 11 2016

A343826 Numbers which are the product of two S-primes (A057948) in exactly one way.

Original entry on oeis.org

25, 45, 65, 81, 85, 105, 117, 145, 153, 165, 169, 185, 189, 205, 221, 245, 261, 265, 273, 285, 289, 297, 305, 333, 345, 357, 365, 369, 377, 385, 429, 445, 465, 477, 481, 485, 493, 505, 513, 533, 545, 549, 561, 565, 605, 609, 621, 629, 637, 645, 657, 665, 685

Offset: 1

Zachary DeStefano, Apr 30 2021

nonn

There exist numbers which are the product of two S-primes in exactly 1, 2, and 3 ways; however, it is unknown if any numbers exist which are the product of two S-primes in exactly 4 ways.

			153 = 9*17 which are both S-primes, and admits no other S-prime factorizations.

Zachary DeStefano, Table of n, a(n) for n = 1..3786

Cf. A054520, A057948, A057949, A057950.

Exactly two ways: A343827. Exactly three ways: A343828.

\\ uses is(n) from A057948
isok(n) = sumdiv(n, d, (d<=n/d) && is(d) && is(n/d)) == 1; \\ Michel Marcus, May 01 2021

a(n) == 1 (mod 4). - Hugo Pfoertner, May 01 2021

A343827 Numbers which are the product of two S-primes (A057948) in exactly two ways.

Original entry on oeis.org

441, 693, 1089, 1197, 1449, 1617, 1881, 1953, 2277, 2541, 2709, 2793, 2961, 3069, 3249, 3381, 3717, 3933, 4221, 4257, 4473, 4557, 4653, 4761, 4977, 5229, 5301, 5841, 5929, 6321, 6417, 6489, 6633, 6741, 6897, 6909, 7029, 7353, 7581, 7821, 8001, 8037, 8217, 8253

Offset: 1

Zachary DeStefano, Apr 30 2021

nonn

First differs from A057950 at a(21)=4473, whereas A057950(21)=4389, which can be represented as the product of two S-primes in exactly 3 ways.

There exist numbers which are the product of two S-primes in exactly 1, 2, and 3 ways; however, it is unknown if any numbers exist which are the product of two S-primes in exactly 4 ways.

			1449=9*161=21*69 which are all S-primes (A057948), and admits no other S-prime factorizations.

Zachary DeStefano, Table of n, a(n) for n = 1..2484

Cf. A054520, A057948, A057949, A057950.

Exactly one way: A343826. Exactly three ways: A343828.

\\ uses is(n) from A057948
isok(n) = sumdiv(n, d, (d<=n/d) && is(d) && is(n/d)) == 2; \\ Michel Marcus, May 01 2021

a(n) == 1 (mod 4). - Hugo Pfoertner, May 01 2021

A343828 Numbers which are the product of two S-primes (A057948) in exactly three ways.

Original entry on oeis.org

4389, 5313, 7161, 9177, 9933, 10857, 12369, 13629, 14421, 14973, 15477, 16401, 17157, 18249, 18753, 19173, 19437, 20769, 22701, 23529, 23541, 23793, 24717, 26733, 26961, 27993, 28329, 28497, 29337, 29469, 30261, 30597, 31521, 32109, 32361, 32637, 33117, 33649

Offset: 1

Zachary DeStefano, Apr 30 2021

nonn

There exist numbers which are the product of two S-primes in exactly 1, 2, and 3 ways.

An S-prime is either a prime of the form 4k+1 or a semiprime of the form (4k+3)*(4m+3). That means the maximum number of prime factors that a number factorizable into two S-primes can have is four (all 4k + 3), and those can be combined into S-primes in at most three distinct ways. - Gleb Ivanov, Dec 07 2021

			9177 = 21*437 = 57*161 = 69*133 which are all S-primes (A057948), and admits no other S-Prime factorizations.
4389 = (3*7)*(11*19) = (3*11)*(7*19) = (3*19)*(7*11); 3,7,11,19 are the smallest primes of the form 4k + 3.

Zachary DeStefano, Table of n, a(n) for n = 1..1014

Cf. A054520, A057948, A057949, A057950.

Exactly one way: A343826. Exactly two ways: A343827.

\\ uses is(n) from A057948
isok(n) = sumdiv(n, d, (d<=n/d) && is(d) && is(n/d)) == 3; \\ Michel Marcus, May 01 2021

a(n) == 1 (mod 4). - Hugo Pfoertner, May 01 2021

cp's OEIS Frontend

A054520 Let S = {1,5,9,13,..., 4n+1, ...} and call p in S an S-prime if p>1 and the only divisors of p in S are 1 and p; sequence gives elements of S that are not S-primes.

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A057948 S-primes: let S = {1,5,9, ... 4i+1, ...}; then an S-prime is in S but is not divisible by any members of S except itself and 1.

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A057949 Numbers with more than one factorization into S-primes. See A054520 and A057948 for definition.

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A343826 Numbers which are the product of two S-primes (A057948) in exactly one way.

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A343827 Numbers which are the product of two S-primes (A057948) in exactly two ways.

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A343828 Numbers which are the product of two S-primes (A057948) in exactly three ways.

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