OFFSET
1,1
COMMENTS
This sequence is infinite, see A189982 and Theorem 4 in Goldston-Graham-Pintz-Yıldırım. - Charles R Greathouse IV, Jul 17 2015
This is a subsequence of A005237, hence a(n) >> n sqrt(log log n) by the Erdős-Pomerance-Sárközy result cited there. - Charles R Greathouse IV, Jul 17 2015
Sequence is not the same as A280074, first deviation is at a(212): a(212) = 2041, A280074(212) = 2024. Number 2024 is the smallest number n such that A007425(n) = A007425(n+1) with different prime signatures of numbers n and n+1 (2024 = 2^3 * 11 * 23, 2025 = 3^4 * 5^2; A007425(2024) = A007425(2025) = 90). Conjecture: also numbers n such that Product_{d|n} tau(d) = Product_{d|n+1} tau(d). - Jaroslav Krizek, Dec 25 2016
LINKS
T. D. Noe, Table of n, a(n) for n = 1..10000
D. A. Goldston, S. W. Graham, J. Pintz, and C. Y. Yıldırım, Small gaps between almost primes, the parity problem, and some conjectures of Erdos on consecutive integers, arXiv:0803.2636 [math.NT], 2008.
Eric Weisstein's MathWorld, Prime Signature
Wikipedia, Prime signature
EXAMPLE
14 = 2^1*7^1 and 15 = 3^1*5^1, so both have prime signature {1,1}. Thus, 14 is a term.
MATHEMATICA
pri[n_] := Sort[ Transpose[ FactorInteger[n]] [[2]]]; Select[ Range[ 2, 1000], pri[#] == pri[#+1] &]
Rest[SequencePosition[Table[Sort[FactorInteger[n][[All, 2]]], {n, 500}], {x_, x_}][[All, 1]]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Aug 28 2017 *)
PROG
(PARI) lista(nn) = for (n=1, nn-1, if (vecsort(factor(n)[, 2]) == vecsort(factor(n+1)[, 2]), print1(n, ", ")); ); \\ Michel Marcus, Jun 10 2015
(Python)
from sympy import factorint
def aupto(limit):
alst, prevsig = [], [1]
for k in range(3, limit+2):
sig = sorted(factorint(k).values())
if sig == prevsig: alst.append(k - 1)
prevsig = sig
return alst
print(aupto(250)) # Michael S. Branicky, Sep 20 2021
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Erich Friedman, Jan 29 2000
STATUS
approved