A178492 - OEIS

A178492

Primitive elements of A006532, i.e., which are not product of earlier terms (> 1) in that sequence.

1, 3, 22, 70, 81, 94, 115, 119, 170, 214, 217, 265, 310, 322, 343, 364, 382, 385, 400, 472, 497, 517, 527, 679, 710, 742, 745, 782, 820, 862, 884, 889, 935, 970, 1004, 1066, 1080, 1174, 1177, 1207, 1219, 1270, 1393, 1426, 1465, 1501, 1566, 1612, 1624, 1645

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OFFSET

1,2

COMMENTS

If a,b are coprime elements of A006532, their product is again in A006532. But there are some a,b which are not coprime and still a,b,a*b are in A006532: First examples are 820*{2860, 3340, 8580, 10020, 11620, ...}.

LINKS

Donovan Johnson, Table of n, a(n) for n = 1..10000

EXAMPLE

The terms a(2)=3 and a(3)=22 are coprime elements of A006532, therefore 3*22=66 is a non-primitive element of A006532, and not listed in this sequence.

PROG

(PARI) for(n=1, 1e4, issquare(sigma(n))||next; fordiv( n, d, d>1||next; d*d>n & break; issquare(sigma(d)) & issquare(sigma(n/d)) & next(2) ); print1(n", "))

CROSSREFS

Sequence in context: A159345 A006532 A274870 * A005288 A143166 A268792

Adjacent sequences: A178489 A178490 A178491 * A178493 A178494 A178495

KEYWORD

nonn

AUTHOR

Farideh Firoozbakht and M. F. Hasler, Jun 10 2010

STATUS

approved