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A118697 Semiprimes with consecutive digits. 2
4, 6, 9, 10, 21, 34, 65, 87, 123, 321, 543, 789, 901, 1234, 4321, 8765, 34567, 56789, 98765, 654321, 876543, 901234, 1234567, 7654321, 7890123, 10987654, 21098765, 567890123, 12345678901, 54321098765, 76543210987, 87654321098 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Digits can be in ascending or descending order. After 9 comes 0.

The number of semiprimes with n consecutive digits: 3,5,5,3,3,3,3,2,1,0,4,0,2,0,2,1,1,1,1,0,1,2,0,1,0,0,2,1,0,0,1,1,1,0,0,0,0,2,1,0,1,0,0,0,2,1,0,0,0,0,..., . - Robert G. Wilson v, Jun 16 2006

LINKS

Table of n, a(n) for n=1..32.

MATHEMATICA

spQ[n_] := Plus @@ Last /@ FactorInteger@n == 2; f[n_] := Block[{u = Range@n, d = Reverse@ Range@n, t = Table[1, {n}]}, Select[ Drop[ Union@ Flatten@ Table[ FromDigits /@ Mod[{u, d} + {i*t, i*t}, 10], {i, 10}], 2], spQ@# &]]; Array[f, 13] // Flatten - Robert G. Wilson v, Jun 16 2006

CROSSREFS

Cf. A001358.

Sequence in context: A108635 A071964 A135257 * A118695 A235108 A215478

Adjacent sequences:  A118694 A118695 A118696 * A118698 A118699 A118700

KEYWORD

base,nonn

AUTHOR

Luc Stevens (lms022(AT)yahoo.com), May 20 2006

EXTENSIONS

More terms from Robert G. Wilson v, Jun 16 2006

STATUS

approved

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Last modified April 6 14:08 EDT 2020. Contains 333276 sequences. (Running on oeis4.)