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 A069154 Treated as strings, the concatenation c of the prime factors of n, in increasing order, is an initial segment of n. Equivalently, n begins with c. 1
 250, 256, 2048, 2176, 2304, 2500, 2560, 2744, 23328, 25000, 25600, 119911, 219488, 236196, 250000, 256000, 262144, 290912, 2097152, 2238728, 2317312, 2359296, 2370816, 2500000, 2560000, 3515625, 3720087, 5117695, 13436683, 21359416, 23592960, 23887872 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All terms listed above have at most three prime factors. Is there an upper bound to the number of prime factors of terms of this sequence? Called "enlightened numbers" by J. Pe. Note that the Mathematica program he provides in his web page erroneously recognizes some numbers as members of the sequence, for example 2239488. The smallest member with 4 prime factors is 2377970784 = 2^5*3^5*7^2*79^2. - Giovanni Resta, Apr 01 2013 LINKS Giovanni Resta, Table of n, a(n) for n = 1..106 (terms < 10^12) Joseph L. Pe, The Enlightened Numbers EXAMPLE The prime factors of 119911 are 11 and 991, which when concatenated yield 11991, an initial segment of 119911. Therefore 119911 is a term of the sequence. MATHEMATICA seq = {}; Do[If[PrimeQ[n] || EvenQ[n] && IntegerDigits[n][[1]] != 2, Continue[]]; p = StringPosition[ToString@n, StringJoin[ToString /@ First /@ FactorInteger@n]]; If[p != {} && p[[1, 1]] == 1, Print[n]; AppendTo[seq, n]], {n, 2, 10^6}]; seq (* Giovanni Resta, Apr 01 2013 *) CROSSREFS Sequence in context: A197400 A225209 A197363 * A045169 A267856 A268269 Adjacent sequences: A069151 A069152 A069153 * A069155 A069156 A069157 KEYWORD base,nonn AUTHOR Joseph L. Pe, Apr 08 2002 EXTENSIONS Corrected and extended by Giovanni Resta, Apr 01 2013 STATUS approved

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Last modified February 1 02:31 EST 2023. Contains 359981 sequences. (Running on oeis4.)