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A242804 Integers n such that each of n, n+1, n+2, n+4, n+5, n+6 is the product of two distinct primes. 6
213, 143097, 194757, 206133, 273417, 684897, 807657, 1373937, 1391757, 1516533, 1591593, 1610997, 1774797, 1882977, 1891761, 2046453, 2051493, 2163417, 2163957, 2338053, 2359977, 2522517, 2913837, 3108201, 4221753 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

A remarkable gap occurs between the initial two members, and the sequence seems to be rather sparse compared to the related A242805.

Here, the first member n of the sextet is the reference, whereas in A068088 the center n+3 is selected as reference. Observe that n+3 must be divisible by the square 4.

All terms are 9 mod 12. - Zak Seidov, Apr 14 2015

From Robert Israel, Apr 15 2015: (Start)

All terms are 33 mod 36.

n in A039833 such that n+4 is in A039833. (End)

From Robert G. Wilson v, Apr 15 2015: (Start)

n is congruent to 33 (mod 36) so one of its factors is 3 and the other is == 11 (mod 12);

n+1 is congruent to 34 (mod 36) so one of its factors is 2 and the other is == 17 (mod 18);

n+2 is congruent to 35 (mod 36) so its factors are == ±1 (mod 6);

n+4 is congruent to 1 (mod 36) so its factors are == ±1 (mod 6);

n+5 is congruent to 2 (mod 36) so one of its factors is 2 and the other is == 1 (mod 18);

n+6 is congruent to 3 (mod 36) so one of its factors is 3 and the other is == 1 (mod 12). (End).

Number of terms < 10^k: 0, 0, 1, 1, 1, 7, 39, 169, 882, 4852, 27479, …,. - Robert G. Wilson v, Apr 15 2015

Or, numbers n such that n, n+1 and n+2 are terms in A175648. - Zak Seidov, Dec 08 2015

LINKS

Zak Seidov and Robert G. Wilson v, Table of n, a(n) for n = 1..10000

EXAMPLE

213=3*71, 214=2*107, 215=5*43, 217=7*31, 218=2*109, 219=3*73.

MAPLE

f:= t -> numtheory:-issqrfree(t) and (numtheory:-bigomega(t) = 2):

select(t -> andmap(f, [t, t+1, t+2, t+4, t+5, t+6]), [seq(36*k+33, k=0..10^6)]); # Robert Israel, Apr 15 2015

MATHEMATICA

fQ[n_] := PrimeQ[n/3] && PrimeQ[(n + 1)/2] && PrimeQ[(n + 5)/2] && PrimeQ[(n + 6)/3] && PrimeNu[{n + 2, n + 4}] == {2, 2} == PrimeOmega[{n + 2, n + 4}]; k = 33; lst = {}; While[k < 10^8, If[fQ@ k, AppendTo[lst, k]]; k += 36]; lst (* Robert G. Wilson v, Apr 14 2015 and revised Apr 15 2015 after Zak Seidov and Robert Israel *)

PROG

(PARI) default(primelimit, 1000M); i=0; j=0; k=0; l=0; m=0; loc=0; lb=2; ub=9*10^9; o=2; for(n=lb, ub, if(issquarefree(n)&&(o==omega(n)), loc=loc+1; if(1==loc, i=n; ); if(2==loc, if(i+1==n, j=n; ); if(i+1<n, loc=1; i=n; ); ); if(3==loc, if(j+1==n, k=n; ); if(j+1<n, loc=1; i=n; ); ); if(4==loc, if(k+2==n, l=n; ); if(k+2<n, loc=1; i=n; ); ); if(5==loc, if(l+1==n, m=n; ); if(l+1<n, loc=1; i=n; ); ); if(6==loc, if(m+1==n, print1(i, ", "); loc=0; ); if(m+1<n, loc=1; i=n))))

(PARI) forstep(x=213, 4221753, 12, if( isprime(x/3) && isprime((x+1)/2) && 2==omega(x+2) && 2==bigomega(x+2) && 2==omega(x+4) && 2==bigomega(x+4) && isprime((x+5)/2) && isprime((x+6)/3), print1(x", "))) \\ Zak Seidov, Apr 14 2015

CROSSREFS

Cf. A242793 (minima for two, three and more prime divisors) and A068088 (arbitrary squarefree integers).

Cf. A242805, A039833, A175648.

Sequence in context: A280898 A180395 A242793 * A076159 A258961 A204486

Adjacent sequences:  A242801 A242802 A242803 * A242805 A242806 A242807

KEYWORD

nonn

AUTHOR

Daniel Constantin Mayer, May 23 2014

STATUS

approved

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Last modified January 17 07:18 EST 2018. Contains 297787 sequences.