login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A254752 Composite numbers such that, in base 10, all their proper prefixes and suffixes represent composites. 6
44, 46, 48, 49, 64, 66, 68, 69, 84, 86, 88, 94, 96, 98, 99, 404, 406, 408, 424, 426, 428, 444, 446, 448, 454, 456, 458, 464, 466, 468, 469, 484, 486, 488, 494, 496, 498, 604, 606, 608, 609, 624, 626, 628, 634, 636, 638, 639, 644, 646, 648, 649, 654, 656, 658, 664, 666, 668, 669, 684, 686, 688, 694, 696, 698, 699, 804, 806, 808, 814, 816, 818, 824, 826, 828 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

A proper prefix (or suffix) of a number m is one which is neither void, nor identical to m.

Alternative definition: Slicing the decimal expansion of the composite number a(n) in any way into two nonempty parts, each part represents a composite number.

This list is infinite because any string of two or more digits selected from {4,6,8} is a member.

It is a subsequence of A254750 and shares with it these properties: Each member of a(n) must start, as well as end, with one of the digits {4,6,8,9}. Every proper prefix of each member a(n) is a member of A202260, and every proper suffix is a member of A254755.

LINKS

Stanislav Sykora, Table of n, a(n) for n = 1..10000

EXAMPLE

6 is not a member because its expansion cannot be sliced in two.

The composite 469 is a member because it is a composite and the slices (4, 69, 46, and 9) are all composites.

PROG

(PARI) isComposite(n) = (n>2)&&(!isprime(n));

slicesIntoComposites(n, b=10) = {my(k=b); if(n<b, return(0); ); while(n\k>0, if(!isComposite(n\k)||!isComposite(n%k), return(0); ); k*=b); return(1); }

isCompositeSlicingIntoComposites(n, b=10) = isComposite(n) && slicesIntoComposites(n, b);

CROSSREFS

Cf. A202260, A254750, A254751, A254753, A254754, A254755.

Sequence in context: A254750 A225619 A121610 * A063837 A178755 A182261

Adjacent sequences:  A254749 A254750 A254751 * A254753 A254754 A254755

KEYWORD

nonn,base

AUTHOR

Stanislav Sykora, Feb 15 2015

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified May 22 16:19 EDT 2017. Contains 286882 sequences.