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A242413 Numbers in whose prime factorization the first differences of indices of distinct primes form a palindrome; fixed points of A242415. 5
1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 13, 16, 17, 18, 19, 21, 23, 24, 25, 27, 29, 30, 31, 32, 36, 37, 41, 43, 47, 48, 49, 53, 54, 59, 60, 61, 63, 64, 65, 67, 70, 71, 72, 73, 79, 81, 83, 89, 90, 96, 97, 101, 103, 107, 108, 109, 113, 120, 121, 125, 127, 128, 131, 133, 137, 139, 140, 144 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Number n is present, if its prime factorization n = p_a^e_a * p_b^e_b * p_c^e_c * ... * p_i^e_i * p_j^e_j * p_k^e_k where a < b < c < ... < i < j < k, satisfies the condition that the first differences of prime indices (a-0, b-a, c-b, ..., j-i, k-j) form a palindrome.

More formally, numbers n whose prime factorization is either of the form p^e (p prime, e >= 0), i.e., one of the terms of A000961, or of the form p_i1^e_i1 * p_i2^e_i2 * p_i3^e_i3 * ... * p_i_{k-1}^e_{i_{k-1}} * p_{i_k}^e_{i_k}, where p_i1 < p_i2 < ... < p_i_{k-1} < p_k are distinct primes (sorted into ascending order) in the prime factorization of n, and e_i1 .. e_{i_k} are their nonzero exponents (here k =  A001221(n) and i_k = A061395(n), the index of the largest prime present), and the indices of primes satisfy the relation that for each index i_j < i_k present, the index i_{k-j} is also present.

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..10000

EXAMPLE

1 is present because it has an empty factorization, so both the sequence of the prime indices and their first differences are empty, and empty sequences are palindromes as well.

12 = 2*2*3 = p_1^2 * p_2 is present, as the first differences (deltas) of prime indices (1-0, 2-1) = (1,1) form a palindrome.

60 = 2*2*3*5 = p_1^2 * p_2 * p_3 is present, as the deltas of prime indices (1-0, 2-1, 3-2) = (1,1,1) form a palindrome.

61 = p_18 is present, as the deltas of prime indices, (18), form a palindrome.

144 = 2^4 * 3^2 = p_1^4 * p_2^2 is present, as the deltas of prime indices (1-0, 2-1) = (1,1) form a palindrome.

Also, any of the cases mentioned in the Example section of A242417 as being present there, are also present in this sequence.

PROG

(Scheme, with Antti Karttunen's IntSeq-library)

(define A242413 (FIXED-POINTS 1 1 A242415))

CROSSREFS

Fixed points of A242415.

Differs from A243068 for the first time at n=36, where a(36)=60, while A243068(36)=61.

Cf. A000961, A242417 (subsequences), A242414, A243058, A242421, A088902, A241912.

Sequence in context: A055201 A213006 A072303 * A243068 A081061 A317589

Adjacent sequences:  A242410 A242411 A242412 * A242414 A242415 A242416

KEYWORD

nonn

AUTHOR

Antti Karttunen, May 31 2014

STATUS

approved

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Last modified April 2 23:47 EDT 2020. Contains 333194 sequences. (Running on oeis4.)