login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A308252 Squarefree composite numbers k with chained prime factors: the last digit of every prime factor is the same as the first digit of the next one. Prime factors must be in ascending order. 2
46, 58, 93, 111, 143, 187, 209, 265, 295, 403, 422, 446, 454, 458, 466, 478, 481, 482, 497, 502, 511, 514, 526, 538, 542, 553, 554, 562, 566, 586, 713, 851, 921, 933, 939, 951, 993, 1011, 1041, 1047, 1059, 1077, 1101, 1111, 1119, 1133, 1137, 1149, 1167, 1177, 1191, 1199 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

This sequence contains all squarefree numbers of A308099.

LINKS

Table of n, a(n) for n=1..52.

EXAMPLE

1426 is such a number because it is a squarefree composite, with prime factors in ascending order 2, 23 and 31 which are chained.

MATHEMATICA

Select[Range@1500, SquareFreeQ@#&&PrimeNu@#>1&&And@@(Last@#[[1]]==First@#[[2]]&/@Partition[IntegerDigits@*First/@FactorInteger@#, 2, 1])&]

PROG

(PARI) isok(n) = {my(f=factor(n)[, 1]); if ((#f <= 1) || !issquarefree(n), return(0)); my(vd=digits(f[1]), d=vd[#vd], vd2, d2); if ((#f <= 1) || !issquarefree(n), return(0)); for (k=2, #f, vd2 = digits(f[k]); d2 = vd2[1]; if (d2 != d, return (0)); vd = vd2; d = vd[#vd]; ); return (1); } \\ Michel Marcus, May 18 2019

CROSSREFS

Cf. A005117, A308099, A308101.

Sequence in context: A326646 A332952 A308099 * A322161 A039424 A043247

Adjacent sequences:  A308249 A308250 A308251 * A308253 A308254 A308255

KEYWORD

nonn,base

AUTHOR

Giorgos Kalogeropoulos, May 17 2019

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
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified October 23 11:46 EDT 2021. Contains 348212 sequences. (Running on oeis4.)