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A046461 Numbers n such that concatenation of numbers from 1 to n is a semiprime. 3
3, 4, 7, 34, 97 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

From Sean A. Irvine, Apr 15 2010: (Start)

5053 and 9706 are definite members of the sequence.

The next potential member is 1051 (a factor is needed to establish membership). (End)

1051 eliminated, next potential member is 1651. - Sean A. Irvine, Nov 11 2014

LINKS

Table of n, a(n) for n=1..5.

P. De Geest, Normal Smarandache Concatenated Numbers, Prime factors from 1 up to n

M. Fleuren, Factors and primes of Smarandache sequences.

M. Fleuren, Smarandache Factors and Reverse factors

C. Rivera, Primes by Listing

EXAMPLE

Sm(691)=1304238680165623831238651513722972177904593843651*C1916, so Sm(691) is not a semiprime and 691 is not a member of this sequence.

CROSSREFS

Cf. A046460.

Sequence in context: A007677 A042773 A042173 * A084590 A041135 A041379

Adjacent sequences:  A046458 A046459 A046460 * A046462 A046463 A046464

KEYWORD

nonn,hard,base,more

AUTHOR

Patrick De Geest, Aug 15 1998.

EXTENSIONS

Simplified definition by Sean A. Irvine, Mar 29 2010

a(5) from Sean A. Irvine, Mar 29 2010

STATUS

approved

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Last modified December 18 07:26 EST 2014. Contains 252098 sequences.