

A281592


Products of three distinct primes p1, p2 and p3 (sphenic numbers) with p1<p2 and p3 is the concatenation of p1 with p2.


0



138, 777, 4642, 10258, 10263, 12207, 13282, 16167, 19762, 30783, 37407, 38482, 46978, 48927, 56127, 60145, 63543, 73767, 81687, 89823, 95367, 95627, 103863, 110905, 115527, 128545, 202705, 208879, 223643, 284119, 324947, 325793, 360151, 395003, 477538, 541163, 558322, 585538, 672199, 673693, 780082, 914551, 1016643
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OFFSET

1,1


LINKS

Table of n, a(n) for n=1..43.


EXAMPLE

10258 is in the sequence because 10258 = 2*23*223 and 223 is the concatenation of 2 with 23.


MATHEMATICA

c[x_, y_] := x 10^IntegerLength[y] + y; upto[mx_] := Sort@ Reap[Block[{p=2, q=3, v=1}, While[v <= mx, While[p < q && (v = p q (r = c[p, q])) <= mx, If[PrimeQ@r, Sow@v]; p = NextPrime[p]]; p=2; q = NextPrime[q]; v = p q c[p, q]]]][[2, 1]]; upto[10^6] (* Giovanni Resta, Apr 14 2017 *)


PROG

(PARI) isok(n) = f = factor(n); ((#f~ == 3) && (vecmax(f[, 2]) == 1) && (f[3, 1] == fromdigits(concat(digits(f[1, 1]), digits(f[2, 1]))))); \\ Michel Marcus, Apr 14 2017


CROSSREFS

Cf. A007304, A133980 (the p3 primes).
Sequence in context: A168531 A260133 A317579 * A213963 A279964 A163693
Adjacent sequences: A281589 A281590 A281591 * A281593 A281594 A281595


KEYWORD

nonn,base


AUTHOR

Peter Weiss, Apr 14 2017


STATUS

approved



