Alphabeta Math
TheoremStatement: Literature-sourcedProof: AI-adaptedSession-authored (Fable 5 assisted)precheck passaudited 2026-08-14
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Standard Maclaurin expansions

Statement

The standard Maclaurin expansions are

11x=n=0xn(x<1),

ex=n=0xnn!,sinx=n=0(1)nx2n+1(2n+1)!,cosx=n=0(1)nx2n(2n)!(xR),

log(1+x)=n=1(1)n+1xnn(1<x1),

and

arctanx=n=0(1)nx2n+12n+1(x<1),

with value π/4 at x=1. For every real α,

(1+x)α=n=0(αn)xn(x<1),

where

(α0)=1,(αn+1)=(αn)αnn+1.

Facts & Assumptions

Given: The functions and power series displayed in the statement, and an arbitrary real parameter α for the generalized-binomial family.

[L1]

The Maclaurin series of a smooth function f is n0f(n)(0)xn/n! (Taylor and Maclaurin series).

[L2]

For x<1, the geometric series satisfies n0xn=1/(1x) (For r<1, k0rk=1/(1r), and for r1 the series diverges).

[L3]

For every real x, exp(x)=n0xn/n!, and e:=exp(1) (The real exponential function and the number e by a power series); and sinx=n0(1)nx2n+1/(2n+1)!, cosx=n0(1)nx2n/(2n)! (Sine and cosine defined by their real power series). The first definition names exp and names e only as exp(1); it does not name ex.

[L4]

For 1<x1, log(1+x)=n1(1)n+1xn/n; at x=1 the sum is log2, and at x=1 the series diverges ([The power series for log(1+x) on (-1,1], including the Abel endpoint](/item/thm-log-one-plus-x-power-series)).

[L5]

For x<1, arctanx=n0(1)nx2n+1/(2n+1), and the series at x=1 converges to π/4 (Principal arctangent: derivative, integral, power series, and the Gregory–Leibniz series).

[L6]

For every real β, xxβ is continuous on (0,) and has derivative βxβ1 there (Continuity and derivatives of positive-base real powers).

[L7]

Factorials satisfy (n+1)!=(n+1)n! (The factorial n! and the falling factorial nk, defined by recursion in N), and for reals u,v and a natural number m the finite binomial theorem is (u+v)m=k=0m(mk)ukvmk (The binomial theorem in R: (x+y)n=k<n+1ι ⁣(nk)xkynk), the coefficients being the natural numbers (mk) read in R through the canonical embedding. The exponents there are the natural-number ones: for every real a, nan is the unique function on N with a0=1 and an+1=ana (Integer powers am).

[L8]

If a sequence has no zero terms and lim supnan+1/an<1, then an converges absolutely (Ratio test: lim supak+1/ak<1 gives absolute convergence and hence convergence, and lim infak+1/ak>1 gives divergence).

[L9]

A real power series may be differentiated term by term at every point inside its radius of convergence, and the differentiated series has the same radius (Inside its radius a real power series may be differentiated term by term, and the differentiated series has the same radius).

[L11]

If a real-valued function is continuous on an order-convex interval and has derivative zero at every interior point, then it is constant (A function continuous on an interval I whose derivative vanishes at every interior point of I is constant on I; consequently two such functions with the same derivative differ by a constant).

[L12]

For naturals kn, (nk)k!(nk)!=n!; consequently, as a real number, (nk)=n!/(k!(nk)!) ((nk)k!(nk)!=n! for kn; hence (nk)k!=nk, the quotient n!/(k!(nk)!) is a natural number, and (nk)=(nnk)).

[L13]

If g is differentiable at a limit point c of its domain and h is differentiable at g(c), itself a limit point of the domain of h, then hg is differentiable at c and (hg)(c)=h(g(c))g(c) (The chain rule, in one line from Carathéodory: if g is differentiable at c and f is differentiable at g(c), then fg is differentiable at c with (fg)(c)=f(g(c))g(c)).

[L15]

A function differentiable at a limit point c of its domain is continuous at c (A function differentiable at c is continuous at c).

[L16]

For a>0 and xR, ax=exp(xloga) (Real powers for positive bases, with the zero-base positive-exponent convention); log is the inverse of exp, so log(expy)=y for every real y and exp(logw)=w for every w>0 (The natural logarithm as the inverse of the exponential function); and for a,b>0 and r,sR, ar+s=aras (The exponent, product, quotient, and iterated-power laws for positive real bases and real exponents).

[L17]

exp:R(0,) is a bijection (The exponential is a continuous bijection from R onto (0,)); in particular every value of exp is positive.

[L18]

A real power series about the centre c is n0an(xc)n, where the powers are those of Integer powers am and convergence is that of Series, partial sums, convergence and the sum, divergence, and the tail series; at x=c it always converges to a0, the n=0 term being a0 because 00=1 and every later term being 0; and its radius of convergence is the supremum, in [0,+], of the r0 such that the series converges absolutely at every x with xc<r (A real power series about a centre, its interval of convergence, and its radius in [0,+]).

[L19]

f(0):=f, and f(j+1):=(f(j)) wherever f(j) is differentiable; f is smooth, that is C, on an interval when every f(j) exists there and is continuous there (Higher derivatives and the classes Ck and C).

[L20]

For naturals n,k, (nk) is the number of k-element subsets of n; in particular (n0)=1 for every n, and (nk)=0 for k>n (The set [A]k of k-element subsets and the binomial coefficient (nk):=[n]k).

[L21]

If ak and bk converge and c is real, then (ak+bk) converges to ak+bk, and cak converges to cak (Convergent series add and scale termwise).

[L22]

A real sequence converges to a real L if and only if its limit inferior and its limit superior are both L (A real sequence converges to LR iff lim infxk=lim supxk=L, and diverges to ± iff both equal ±).

[L23]

If xkx and yky then xk+ykx+y, cxkcx, xkykxy and xkykxy (Algebra of limits: sums, scalar multiples, products and quotients); and for every ε>0 there is a natural n1 with 1/n<ε (For every ε>0 in a complete ordered field there is a natural n1 with 1/n<ε).

[L24]

A real power series an(xc)n of radius R converges absolutely at every x with xc<R and diverges at every x with xc>R (A real power series converges absolutely inside its radius and diverges outside it, while either behaviour may occur at an endpoint).

Proof

technique · direct
1.1

The geometric, sine, cosine, logarithmic, and inverse-tangent identities, with exactly the displayed domains and endpoint assertions, are [L2]–[L5]. The exponential identity needs one further move, because the statement writes ex, the real power of the base e in the sense of [L16], while [L3] defines exp and defines e only as exp(1). By [L17] every value of exp is positive, so e=exp(1)>0 and [L16] assigns ex the value exp(xloge). Since log is the inverse of exp [L16], loge=log(exp1)=1, so ex=exp(x1)=exp(x)(xR), and the series [L3] gives for exp(x) is therefore the series of ex.

L2L3L4L5L16L17algebra
1.2

Suppose f(x)=n0anxn for x<R with R>0; this is a power series about 0 in the sense of [L18], and the hypothesis is exactly that it converges, with sum f(x), at every x with x<R. Its radius of convergence ρ is therefore at least R: by [L24] a power series diverges at every point farther from its centre than its radius, so a point of convergence x has xρ, and letting x run over [0,R) gives ρR. Every point of (R,R) thus lies strictly inside the radius, so by [L9] the sum is differentiable there and its derivative is again a power series of radius ρ; induction on k, using (n+1)!=(n+1)n! from [L7] at each step, gives f(k)(x)=n0(n+k)!n!an+kxn(x<R) for every k0, each again a power series about 0 of radius ρ. Evaluating at the centre, [L18] gives f(k)(0)=k!ak. Every f(k) is differentiable on (R,R), hence continuous there by [L15], so f is smooth on (R,R) in the sense of [L19] and [L1] applies at a=0: the Maclaurin series of f is k0f(k)(0)xk/k!=k0akxk. A power series representing f near 0 is therefore its Maclaurin series.

L1L7L9L15L18L19L24algebra
1.3

Fix αR and define c0=1 and cn+1=cn(αn)/(n+1) for n0.

L7choose
1.4

On (1,1), the function x(1+x)α is differentiable and has derivative α(1+x)α1. The inner map x1+x is a polynomial, so by [L14] it is differentiable with derivative 1 at every point of (1,1), each of which is a limit point of (1,1); its value 1+x lies in (0,) and is a limit point of (0,), where by [L6] the outer map uuα is differentiable with derivative αuα1. The chain rule [L13] therefore gives the composite derivative α(1+x)α11.

L6L13L14algebra
2.1

If α=m is a nonnegative integer, the recurrence of step 1.3 gives cn=(mn) for 0nm and cn=0 for n>m. Indeed c0=1=(m0) by [L20]; and if n<m and cn=(mn)=m!/(n!(mn)!) as a real number by [L12], then, since (n+1)!=(n+1)n! and (mn)!=(mn)(mn1)! with mn1, cn+1=m!n!(mn)!mnn+1=m!(n+1)!(m(n+1))!=(mn+1); while cm+1=cm(mm)/(m+1)=0, after which the recurrence keeps every term 0. Taking u=x and v=1 in the finite binomial theorem of [L7] then gives (1+x)m=(x+1)m=k=0m(mk)xk1mk=k=0m(mk)xk=n0cnxn, using 1j=1 for every natural j, which the recursion 10=1, 1j+1=1j1 of [L7] gives by induction, and using cn=0 for n>m. The exponent in (1+x)m is there the natural-number one of [L7]. For x<1 that value is also the real power (1+x)m of [L16]: since 1+x>0, the real powers (1+x)n with nN satisfy (1+x)0=exp(0log(1+x))=exp0=1, the value exp0=1 being the value at the centre of the series [L3] gives for exp, by [L18]; and, by the addition law of [L16] together with (1+x)1=exp(log(1+x))=1+x, also (1+x)n+1=(1+x)n(1+x)1=(1+x)n(1+x). That is the recursion determining the natural-number powers in [L7], so the two readings of (1+x)m agree.

L3L7L12L16L18L20step 1.3algebra
2.2

Suppose α is not a nonnegative integer and 0<x<1. Then, by step 1.3, c0=1 and no factor αn of the recurrence vanishes, so every cn is nonzero, and xn0; the ratios qn:=cn+1xn+1/(cnxn)=xαn/(n+1) are therefore defined. For every n>α one has αn=nα and hence qn=x(1α+1n+1), so, since 1/(n+1)0 by [L23] and convergence of a sequence is a condition on its tails, [L23] gives qnx. By [L22] the limit superior is that same limit, so lim supnqn=x<1 and [L8] makes n0cnxn converge absolutely.

L8L22L23step 1.3algebra
2.3

Thus the six series in step 1.1 are precisely the asserted Maclaurin expansions; the logarithmic and inverse-tangent endpoint values are values of the same series, not claims of an open interval beyond its radius.

step 1.1step 1.2
3.1

Therefore, for every real α, the power series B(x):=n0cnxn built from step 1.3 converges absolutely at every x with x<1: when α=m is a nonnegative integer every term past index m vanishes by step 2.1 and the series is a finite sum; at x=0 it converges to c0 by [L18]; and every remaining case is step 2.2. Its radius of convergence in the sense of [L18] is therefore at least 1.

L18step 1.3step 2.1step 2.2algebra
4.1

By [L9] and step 3.1, B is differentiable on (1,1) with B(x)=n0(n+1)cn+1xn, again a power series of radius at least 1. The recurrence of step 1.3 gives (n+1)cn+1=(αn)cn, so B(x)=n0(αn)cnxn for x<1. Both n0αcnxn and n0ncnxn=xn0(n+1)cn+1xn converge there, the first by step 3.1 and the second because it is xB(x), so [L21] splits the sum termwise into αB(x)xB(x). Hence B(x)=αB(x)xB(x), that is (1+x)B(x)=αB(x), for x<1.

L9L21step 1.3step 3.1algebra
5.1

For G(x):=(1+x)αB(x), both factors are differentiable on (1,1) by step 1.4 and step 4.1, so the product rule gives G(x)=α(1+x)α1B(x)+(1+x)αB(x) there. On (1,1) the base 1+x is positive, so (1+x)1=exp(log(1+x))=1+x and the addition law at the real exponents α1 and 1 gives (1+x)α1(1+x)=(1+x)α, both by [L16]. Multiplying the displayed derivative by 1+x and using (1+x)B(x)=αB(x) from step 4.1 therefore gives (1+x)G(x)=α(1+x)αB(x)+α(1+x)αB(x)=0; since 1+x0, G(x)=0 throughout (1,1).

L10L16step 1.4step 4.1algebra
6.1

Step 5.1 makes G differentiable at every point of (1,1), and every such point is a limit point of (1,1), so [L15] makes G continuous on (1,1).

L15step 5.1
7.1

The interval (1,1) is order-convex, so [L11] and step 6.1 make G constant there, and its constant value is G(0)=1αB(0). Here B(0)=c0 by [L18], the value of a power series at its centre being its constant coefficient, and c0=1 by step 1.3. And 1α is a real power of the base 1>0, so [L16] makes it exp(αlog1), where exp0=1 is the value at the centre of the series [L3] gives for exp, by [L18], hence log1=log(exp0)=0 and 1α=exp0=1. The constant value is therefore 1, that is (1+x)αB(x)=1 for every x<1. Multiplying by the real power (1+x)α and using the addition law of [L16] at the real exponents α and α, which gives (1+x)α(1+x)α=(1+x)0=exp(0log(1+x))=1, yields B(x)=(1+x)α for every x<1.

L3L11L16L18step 1.3step 6.1algebra
8.1

By step 1.2, applied to B on (1,1), this is the Maclaurin expansion of (1+x)α; its coefficients are the recursively defined numbers (αn)=cn of the statement. That notation extends the library's binomial coefficient rather than clashing with it: when α=m is a nonnegative integer, step 2.1 identifies cn with the count (mn) of [L20]. The argument makes no assertion at x=1 or x=1.

L1L20step 1.2step 2.1step 3.1step 7.1
9.1

Combining steps 2.3 and 8.1 proves all the displayed expansions with no additional endpoint claims.

step 2.3step 8.1

Remarks

Which symbol is which. Three symbols in the statement name objects the library builds separately, and each is matched to its own definition rather than to a near neighbour.

ex is the real power of Real powers for positive bases, with the zero-base positive-exponent convention, not the series that defines exp: The real exponential function and the number e by a power series defines exp and defines e only as exp(1). Step 1.1 supplies the bridge, from loge=1.

The exponent in (1+x)α is real, so that power too is exp(αlog(1+x)), whereas the exponent in the finite binomial theorem is a natural number and the (1+x)m appearing there is the integer power of Integer powers am. Step 2.1 proves the two readings agree on (1,1); without that they are different functions with the same name.

For real α the symbol (αn) is defined by the recurrence in the statement, while (nk) elsewhere in the library is a count (The set [A]k of k-element subsets and the binomial coefficient (nk):=[n]k). Step 2.1 proves the recurrence reproduces the count at a nonnegative integer α, so the notation extends rather than overloads.

Two power conventions coexist here without conflict, and it is worth saying which is used where. The integer power fixes 00=1, which is what makes a power series equal its constant coefficient at the centre; the real power leaves 00 undefined and is only ever applied above to bases 1+x>0, 1, and e.

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