A070647 Largest prime factor of sequence of numbers of the form p*q (p, q distinct primes).

3, 5, 7, 5, 7, 11, 13, 11, 17, 7, 19, 13, 23, 17, 11, 19, 29, 31, 13, 23, 37, 11, 41, 17, 43, 29, 13, 31, 47, 19, 53, 37, 23, 59, 17, 61, 41, 43, 19, 67, 47, 71, 13, 29, 73, 31, 79, 53, 23, 83, 59, 89, 61, 37, 17, 97, 67, 101, 29, 41, 103, 19, 71, 107, 43, 31, 109, 73, 17

Offset

1,1

Links

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Formula

a(n) = P(A006881(n)) where P(x) = A006530(x) is the largest prime factor of x.

a(n) = A006881(n)/A096916(n). - Amiram Eldar, Oct 28 2024

Example

6 = 2*3 is the first number of the form p*q (p, q distinct primes) hence a(1) = 3.

Mathematica

f[n_]:=Last/@FactorInteger[n]=={1, 1}; f1[n_]:=Min[First/@FactorInteger[n]]; f2[n_]:=Max[First/@FactorInteger[n]]; lst={}; Do[If[f[n], AppendTo[lst, f2[n]]], {n, 0, 6!}]; lst (* Vladimir Joseph Stephan Orlovsky, Apr 10 2010 *)

Prog

(Haskell)
a070647 = a006530 . a006881 -- Reinhard Zumkeller, Sep 23 2011
(PARI) go(x)=my(v=List()); forprime(p=2, sqrtint(x\1), forprime(q=p+1, x\p, listput(v, [p*q, q]))); apply(v->v[2], vecsort(Vec(v), 1)) \\ Charles R Greathouse IV, Sep 14 2015

Crossrefs

Cf. A006530, A006881, A084127, A096916, A195758.

Sequence in context: A321784 A376833 A225889 * A070949 A222579 A141574

Adjacent sequences: A070644 A070645 A070646 * A070648 A070649 A070650

Keyword

easy,nonn

Author

Benoit Cloitre, May 13 2002

Status

approved