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A046461 Numbers n such that concatenation of numbers from 1 to n is a semiprime. 5
3, 4, 7, 34, 97 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

From Sean A. Irvine, Apr 15 2010, updated Oct 08 2015: (Start)

5053 and 9706 are definite members of the sequence.

The next potential member is 1651.

A007908(1651) is composite, but has no known prime factor, and its least prime factor likely has at least 45 digits. (End)

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

A007908(691)=1304238680165623831238651513722972177904593843651*C1916, so A007908(691) is not a semiprime and 691 is not a member of this sequence.

CROSSREFS

Cf. A007908, A046460.

Sequence in context: A042773 A042173 A317811 * A084590 A279920 A041135

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 September 22 20:30 EDT 2018. Contains 315270 sequences. (Running on oeis4.)