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A114874 Numbers representable in exactly two ways as (p-1)*p^e (where p is a prime and e >= 0) in ascending order. 5
2, 4, 6, 16, 18, 42, 100, 156, 162, 256, 486, 1458, 2028, 4422, 6162, 14406, 19182, 22650, 23548, 26406, 37056, 39366, 62500, 65536, 77658, 113232, 121452, 143262, 208392, 292140, 342732, 375156, 412806, 527802, 564898, 590592, 697048, 843642 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Numbers that are one less than a prime number and of the form (p-1)*p^e for some prime p and e > 0. - Jianing Song, Apr 13 2019

LINKS

Jianing Song, Table of n, a(n) for n = 1..162 (all terms below 10^8)

EXAMPLE

6 is a member because 6 = (3-1)*3^1 = (7-1)*7^0 and 3 and 7 are primes.

MATHEMATICA

s = Split@Sort@Flatten@Table[(Prime[n] - 1)Prime[n]^k, {n, 68000}, {k, 0, 16}]; Union@Flatten@Select[s, Length@# == 2 &] (* Robert G. Wilson v, Jan 05 2006 *)

PROG

(PARI) isA114874(n) = if(n>1, my(v=factor(n), d=#v[, 1], p=v[d, 1], e=v[d, 2]); (isprime(n+1) && n==(p-1)*p^e), 0) \\ Jianing Song, Apr 13 2019

CROSSREFS

Cf. A114871, A114873.

Sequence in context: A096174 A096173 A287681 * A100361 A259939 A069654

Adjacent sequences:  A114871 A114872 A114873 * A114875 A114876 A114877

KEYWORD

nonn

AUTHOR

Franz Vrabec, Jan 03 2006

EXTENSIONS

a(13)-a(38) from Robert G. Wilson v, Jan 05 2006

STATUS

approved

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Last modified November 15 21:37 EST 2019. Contains 329168 sequences. (Running on oeis4.)